P. Busselen - Academia.edu (original) (raw)
Papers by P. Busselen
Comparative Biochemistry and Physiology Part A: Physiology, 1971
Several protein fractions, which are electrophoretically identical to serum proteins, were found ... more Several protein fractions, which are electrophoretically identical to serum proteins, were found in the egg extracts of Car&us maenas, Eriotheir sine&s and Portunus holsatus. However, their relative concentrations were very different from those found in serum.
Comparative Biochemistry and Physiology, 1970
Nature: New biology, Jan 9, 1973
Archives internationales de pharmacodynamie et de thérapie, 1981
Pfl�gers Archiv European Journal of Physiology, 1985
Isolated Langendorff perfused rat hearts were used to study changes in the Ca, Na and K content, ... more Isolated Langendorff perfused rat hearts were used to study changes in the Ca, Na and K content, contractile force and the loss of cellular material during the Ca paradox. Five minutes perfusion with Ca-free solution containing 1 mM EGTA, followed by 10 min of reperfusion in 1.8 mM Ca causes irreversible contracture, K loss, increase in Na and Ca and a massive release of myoglobin and other cellular material into the perfusate (the calcium paradox). During the Ca-free perfusion the ventricles gain Na but the K content decreases slightly. The size of the Na gain appears to depend upon the buffer used and is larger in bicarbonate than in Tris. When HCO3- or H2PO4- ions are omitted from the bathing solution (in Tris, HEPES, or TES buffered salines) the adverse effects of Ca readmission are reduced. Tris buffer gives the best protection. Metabolic inhibition with FCCP (5 X 10(-7) M), or with CN-(2 X 10(-3) M) together with iodoacetic acid (2 X 10(-3) M), decreases Ca uptake during the Ca paradox and inhibits the release of cellular material. In both cases a contracture is observed. Ruthenium red (10(-4) M) does not inhibit the Ca readmission contracture but reduces the release of cellular material and the gain of Ca and Na. The results suggest that the loss of cellular constituents during the calcium paradox, is related to an active uptake of Ca by the mitochondria and may lead to massive changes in the cellular ion concentration, during Ca-repletion.
Pfl�gers Archiv European Journal of Physiology, 1987
Reduction of the Na concentration in the Ca-free perfusion solution reduces the amount of myoglob... more Reduction of the Na concentration in the Ca-free perfusion solution reduces the amount of myoglobin released by the cells when Ca is readmitted if sucrose is used to replace NaCl under mild hypothermia. When salts like cholinechloride or LiCl are used instead of sucrose, no protection is seen at any temperature. The temperature threshold above which myoglobin loss sharply increases is lowered by prolonged Ca depletion or by the addition of EGTA to the Ca-free solution. Protection by sucrose does not occur in the presence of EGTA. An increase of cell Na induced by strophanthidin during the Ca depletion phase has no effect on myoglobin release. The exponential decline in twitch tension in the early phase of Ca deprivation has the same half-live (T1/2) for Ca-free solutions containing 145 mM Na or 35 mM Na (110 mM Li or choline), but its T1/2 is prolonged if sucrose is used to replace NaCl. When 5 mM EGTA is added to the Ca-free solutions, the T1/2 is shortened and is not changed by the replacement of NaCl with sucrose. The rate of washout of Ca within the first 20 s of Ca depletion has a similar time course in a normal Na or in a Li and low Na solution. In a sucrose and low Na solution the rate of the Ca efflux is reduced. The addition of EGTA increases this rate and abolishes the slowing effect of a sucrose and low Na solution. Therefore myoglobin release during the Ca paradox does not depend on the Na gradient across the sarcolemma.(ABSTRACT TRUNCATED AT 250 WORDS)
Archives of Biochemistry and Biophysics, 1970
Archives internationales de physiologie et de biochimie, 1979
The Journal of physiology, 1982
1. (45)Ca fluxes were studied in normal and potassium-depolarized goldfish ventricles as a functi... more 1. (45)Ca fluxes were studied in normal and potassium-depolarized goldfish ventricles as a function of the external Na concentration. Some of the experiments were also performed on guinea-pig auricles.2. When the external K concentration was increased from 5.4 to 142 mM, keeping osmolarity constant by adding 137 mM-Li or choline (hyperosmotically) to the low K solution, the (45)Ca efflux was reversibly inhibited, whereas the [(3)H]sucrose efflux was unaffected.3. Goldfish ventricles, which have been depolarized with 142 mM-K for 100 min, repolarized within 20 min, from ca. -15 mV to ca. -70 mV, following the application of 5.4 mM-K. This repolarization was independent of the presence of external Na. During the repolarization the (45)Ca efflux was reactivated. This reactivation, however, depended on the external Na concentration. Comparable results were obtained in guinea-pig atria.4. A similar repolarization and Na-dependent reactivation of (45)Ca efflux was obtained in goldfish ven...
Journal of Molecular and Cellular Cardiology, 1991
Journal of Molecular and Cellular Cardiology, 1991
Journal of Molecular and Cellular Cardiology, 1991
Polyamine levels were measured by means of high-performance liquid chromatography in Langendorff-... more Polyamine levels were measured by means of high-performance liquid chromatography in Langendorff-perfused rat hearts subjected to the calcium paradox protocol. The concentrations of putrescine, spermidine and spermine did not change significantly during calcium-free perfusion but decreased when calcium was readmitted. This decrease was due to membrane disruption and release of the polyamines into the coronary effluent. The sum of released and remaining spermidine exceeded the concentration of spermidine in control hearts, but, for spermine, this sum was lower than the control level. The addition of 0.5 mM EGTA to the calcium-free solution raised the myocardial concentrations of putrescine and spermidine and enhanced the net increase of spermidine on calcium repletion. DL-alpha-Difluoromethylornithine (DFMO) inhibited these increases and lowered the putrescine level during all perfusion stages. External polyamines had a negative inotropic effect and inhibited the loss of myoglobin on calcium repletion (order of effectiveness: spermine greater than spermine greater than putrescine). Inhibition of contractions by the combined action of verapamil and ryanodine or by potassium depolarization did not prevent myoglobin loss. External polyamines had no effect on high K/low Na contractures, which were mediated mainly by Na-Ca exchange. Calcium-free perfusion in the presence of 0.5 to 1 mM EGTA improved the membrane protection by polyamines or by diamines and analogues, like ornithine, 1,3-diaminopropane, or DFMO, which, in the absence of EGTA, gave no clear protection. It is concluded that calcium depletion and repletion influences myocardiaal polyamine concentrations by (1) membrane disruption and release of polyamines into the coronary effluent, and (2) probably by a stimulation of ornithine decarboxylase activity. The changes in polyamine concentrations do not seem to have any causal role in calcium overload and cell death. Exogenous polyamines protect against membrane damage.
Archives internationales de physiologie et de biochimie, 1975
The American journal of physiology, 1984
Hearts that have been perfused in low calcium fluids suffer, on return to normal calcium solution... more Hearts that have been perfused in low calcium fluids suffer, on return to normal calcium solutions, an impairment of function which can be irreversible-- the "calcium paradox." In hypothermic mammalian, amphibian, and fish heart the strong contracture, which is a typical first stage in the development of the calcium paradox, is reversible and appears to depend on a large rise in intracellular Na concentration ([Na]i), which occurs during the period of Ca deprivation. This rise is mainly due to a maintained inward Na flux through the Ca channels and causes a depolarization of the membrane potential, which stabilizes at about -20 mV. In frog atrial muscle if the membrane potential is clamped to values more negative than -50 mV during the period of Ca deprivation, no contracture develops on the restoration of the extracellular Ca concentration ([Ca]o). In all tissues the depolarization, the rise in [Na]i, and the Ca addition contracture are blocked by Ca channel blockers, ant...
The Journal of Physiology, 1978
International Journal of Evolutionary Biology, 2012
We present a comprehensive phylogenetic analysis of the Utaka, an informal taxonomic group of cic... more We present a comprehensive phylogenetic analysis of the Utaka, an informal taxonomic group of cichlid species from Lake Malawi. We analyse both nuclear and mtDNA data from five Utaka species representing two (Copadichromis and Mchenga) of the three genera within Utaka. Within three of the five analysed species we find two very divergent mtDNA lineages. These lineages are widespread and occur sympatrically in conspecific individuals in different areas throughout the lake. In a broader taxonomic context including representatives of the main groups within the Lake Malawi cichlid fauna, we find that one of these lineages clusters within the non-Mbuna mtDNA clade, while the other forms a separate clade stemming from the base of the Malawian cichlid radiation. This second mtDNA lineage was only found in Utaka individuals, mostly within Copadichromis sp. "virginalis kajose" specimens. The nuclear genes analysed, on the other hand, did not show traces of divergence within each species. We suggest that the discrepancy between the mtDNA and the nuclear DNA signatures is best explained by a past hybridisation event by which the mtDNA of another species introgressed into the ancestral Copadichromis sp. "virginalis kajose" gene pool.
International Journal of Evolutionary Biology, 2011
Peripheral isolated populations may undergo rapid divergence from the main population due to vari... more Peripheral isolated populations may undergo rapid divergence from the main population due to various factors such as a bottleneck or a founder effect followed by genetic drift or local selection pressures. Recent populations of two economically important Copadichromis species in Lake Malombe, a satellite lake of Lake Malawi, were neither genetically nor morphometrically distinct from their source populations in the main lake. Evidence was found for a founder effect which had a different impact on the genetic composition of the two species. In addition, the increased fishing pressure in Lake Malombe may have led to a reduction of the body sizes of both species.
Journal of Molecular and Cellular Cardiology, 1986
- 18, [241][242][243][244][245][246][247][248][249][250][251][252][253][254]. The intracellul... more 1986) 18, [241][242][243][244][245][246][247][248][249][250][251][252][253][254]. The intracellular sodium content of cardiac cells in fish and amphibia, measured with either an isotope technique or with sodium-sensitive micro-electrodes, rises steeply from around 15 mmol/l in calciumcontaining solution to as much as 70 mmol/l, during exposure to a Ca 2 +-free solution. This increase is associated with the development of spontaneous and prolonged action potentials so that the membrane may stabilise around --20 mV. On reperfusion with calcium-containing medium the membrane repolarises before a strong contracture develops. Inhibition of the Na-pump increases both the sodium gain and the subsequent calcium re-admission tension. A number of agents e.g. divalent cations, anti-arrhythmic drugs, local anaesthetics and Ca-channel blockers are able to prevent the development of the contracture but only if they are present during the calcium-free perfusion. They also inhibit the development of spontaneous electrical activity and the rise in Na i . The calcium re-admission contracture can be blocked in amphibian preparations voltage clamped around the resting potential during low calcium perfusion. From the known pharmacological action of these agents and the voltage and time dependence of the calcium channel, it is concluded that during calcium depletion, the prolongation of the action potentials is associated with a sustained entry ofNa + via the Ca-channels which leads to the rise in Na i . Once Na i has risen, these agents with the exception of Mn 2+, a known inhibitor of the Na/Ca exchange, are unable to prevent the development of the contracture. This suggests that the re-admission contracture follows calcium uptake by way of the Na/Ca exchange.
Journal of Molecular and Cellular Cardiology, 1991
Journal of Molecular and Cellular Cardiology, 1991
In rat papillary muscle, rapid cooling causes membrane depolarization which initiates action pote... more In rat papillary muscle, rapid cooling causes membrane depolarization which initiates action potentials that lead to a contraction. This rapid cooling contraction (RCC) can be blocked by TTX, Mn2+, Ni2+ or high K+ superfusion. In the presence of caffeine (0.5-1 mM), the rapid cooling contracture (caffeine-RCC) has an amplitude similar to that of a twitch elicited by field stimulation at 37 degrees C, but is not inhibited by these agents. As the caffeine-RCC appears to be independent of membrane depolarization and Ca influx but can be inhibited by increasing the bathing caffeine concentration to 20 mM, we consider that the amplitude of this contracture gives a good indication of the calcium content of the sarcoplasmic reticulum (SR). In Tyrode containing 1.8 mM Ca an increased stimulus frequency leads to a negative force staircase which is paralleled by a similar decrease in the amplitude of the caffeine-RCC. These effects are lost if the bathing Ca is reduced (0.18-0.45 mM) in a way which can be reversed by isoproterenol (100 nM). In verapamil (2 microM), however whilst the twitch responses may show a steeper dependence upon stimulus frequency, the negative frequency dependence of the caffeine-RCC is also lost. Low external Na+ also inhibits the frequency dependent reduction of the caffeine-RCC. The results suggest that if the amplitude of the caffeine-RCC is a good indication of the SR calcium content, then this Ca store is related reciprocally to membrane Ca current where activation of the Ca channels leads to a depletion of the store whereas inhibition of membrane Ca channels leads to a filling of the Ca store. We propose that on stimulation the size of the Ca influx determines the fraction of Ca released from the SR. This released Ca may be partially extruded from the cell by way of the Na/Ca exchange which acts in competition with the re-uptake mechanism of the SR to control SR Ca content.
Comparative Biochemistry and Physiology Part A: Physiology, 1971
Several protein fractions, which are electrophoretically identical to serum proteins, were found ... more Several protein fractions, which are electrophoretically identical to serum proteins, were found in the egg extracts of Car&us maenas, Eriotheir sine&s and Portunus holsatus. However, their relative concentrations were very different from those found in serum.
Comparative Biochemistry and Physiology, 1970
Nature: New biology, Jan 9, 1973
Archives internationales de pharmacodynamie et de thérapie, 1981
Pfl�gers Archiv European Journal of Physiology, 1985
Isolated Langendorff perfused rat hearts were used to study changes in the Ca, Na and K content, ... more Isolated Langendorff perfused rat hearts were used to study changes in the Ca, Na and K content, contractile force and the loss of cellular material during the Ca paradox. Five minutes perfusion with Ca-free solution containing 1 mM EGTA, followed by 10 min of reperfusion in 1.8 mM Ca causes irreversible contracture, K loss, increase in Na and Ca and a massive release of myoglobin and other cellular material into the perfusate (the calcium paradox). During the Ca-free perfusion the ventricles gain Na but the K content decreases slightly. The size of the Na gain appears to depend upon the buffer used and is larger in bicarbonate than in Tris. When HCO3- or H2PO4- ions are omitted from the bathing solution (in Tris, HEPES, or TES buffered salines) the adverse effects of Ca readmission are reduced. Tris buffer gives the best protection. Metabolic inhibition with FCCP (5 X 10(-7) M), or with CN-(2 X 10(-3) M) together with iodoacetic acid (2 X 10(-3) M), decreases Ca uptake during the Ca paradox and inhibits the release of cellular material. In both cases a contracture is observed. Ruthenium red (10(-4) M) does not inhibit the Ca readmission contracture but reduces the release of cellular material and the gain of Ca and Na. The results suggest that the loss of cellular constituents during the calcium paradox, is related to an active uptake of Ca by the mitochondria and may lead to massive changes in the cellular ion concentration, during Ca-repletion.
Pfl�gers Archiv European Journal of Physiology, 1987
Reduction of the Na concentration in the Ca-free perfusion solution reduces the amount of myoglob... more Reduction of the Na concentration in the Ca-free perfusion solution reduces the amount of myoglobin released by the cells when Ca is readmitted if sucrose is used to replace NaCl under mild hypothermia. When salts like cholinechloride or LiCl are used instead of sucrose, no protection is seen at any temperature. The temperature threshold above which myoglobin loss sharply increases is lowered by prolonged Ca depletion or by the addition of EGTA to the Ca-free solution. Protection by sucrose does not occur in the presence of EGTA. An increase of cell Na induced by strophanthidin during the Ca depletion phase has no effect on myoglobin release. The exponential decline in twitch tension in the early phase of Ca deprivation has the same half-live (T1/2) for Ca-free solutions containing 145 mM Na or 35 mM Na (110 mM Li or choline), but its T1/2 is prolonged if sucrose is used to replace NaCl. When 5 mM EGTA is added to the Ca-free solutions, the T1/2 is shortened and is not changed by the replacement of NaCl with sucrose. The rate of washout of Ca within the first 20 s of Ca depletion has a similar time course in a normal Na or in a Li and low Na solution. In a sucrose and low Na solution the rate of the Ca efflux is reduced. The addition of EGTA increases this rate and abolishes the slowing effect of a sucrose and low Na solution. Therefore myoglobin release during the Ca paradox does not depend on the Na gradient across the sarcolemma.(ABSTRACT TRUNCATED AT 250 WORDS)
Archives of Biochemistry and Biophysics, 1970
Archives internationales de physiologie et de biochimie, 1979
The Journal of physiology, 1982
1. (45)Ca fluxes were studied in normal and potassium-depolarized goldfish ventricles as a functi... more 1. (45)Ca fluxes were studied in normal and potassium-depolarized goldfish ventricles as a function of the external Na concentration. Some of the experiments were also performed on guinea-pig auricles.2. When the external K concentration was increased from 5.4 to 142 mM, keeping osmolarity constant by adding 137 mM-Li or choline (hyperosmotically) to the low K solution, the (45)Ca efflux was reversibly inhibited, whereas the [(3)H]sucrose efflux was unaffected.3. Goldfish ventricles, which have been depolarized with 142 mM-K for 100 min, repolarized within 20 min, from ca. -15 mV to ca. -70 mV, following the application of 5.4 mM-K. This repolarization was independent of the presence of external Na. During the repolarization the (45)Ca efflux was reactivated. This reactivation, however, depended on the external Na concentration. Comparable results were obtained in guinea-pig atria.4. A similar repolarization and Na-dependent reactivation of (45)Ca efflux was obtained in goldfish ven...
Journal of Molecular and Cellular Cardiology, 1991
Journal of Molecular and Cellular Cardiology, 1991
Journal of Molecular and Cellular Cardiology, 1991
Polyamine levels were measured by means of high-performance liquid chromatography in Langendorff-... more Polyamine levels were measured by means of high-performance liquid chromatography in Langendorff-perfused rat hearts subjected to the calcium paradox protocol. The concentrations of putrescine, spermidine and spermine did not change significantly during calcium-free perfusion but decreased when calcium was readmitted. This decrease was due to membrane disruption and release of the polyamines into the coronary effluent. The sum of released and remaining spermidine exceeded the concentration of spermidine in control hearts, but, for spermine, this sum was lower than the control level. The addition of 0.5 mM EGTA to the calcium-free solution raised the myocardial concentrations of putrescine and spermidine and enhanced the net increase of spermidine on calcium repletion. DL-alpha-Difluoromethylornithine (DFMO) inhibited these increases and lowered the putrescine level during all perfusion stages. External polyamines had a negative inotropic effect and inhibited the loss of myoglobin on calcium repletion (order of effectiveness: spermine greater than spermine greater than putrescine). Inhibition of contractions by the combined action of verapamil and ryanodine or by potassium depolarization did not prevent myoglobin loss. External polyamines had no effect on high K/low Na contractures, which were mediated mainly by Na-Ca exchange. Calcium-free perfusion in the presence of 0.5 to 1 mM EGTA improved the membrane protection by polyamines or by diamines and analogues, like ornithine, 1,3-diaminopropane, or DFMO, which, in the absence of EGTA, gave no clear protection. It is concluded that calcium depletion and repletion influences myocardiaal polyamine concentrations by (1) membrane disruption and release of polyamines into the coronary effluent, and (2) probably by a stimulation of ornithine decarboxylase activity. The changes in polyamine concentrations do not seem to have any causal role in calcium overload and cell death. Exogenous polyamines protect against membrane damage.
Archives internationales de physiologie et de biochimie, 1975
The American journal of physiology, 1984
Hearts that have been perfused in low calcium fluids suffer, on return to normal calcium solution... more Hearts that have been perfused in low calcium fluids suffer, on return to normal calcium solutions, an impairment of function which can be irreversible-- the "calcium paradox." In hypothermic mammalian, amphibian, and fish heart the strong contracture, which is a typical first stage in the development of the calcium paradox, is reversible and appears to depend on a large rise in intracellular Na concentration ([Na]i), which occurs during the period of Ca deprivation. This rise is mainly due to a maintained inward Na flux through the Ca channels and causes a depolarization of the membrane potential, which stabilizes at about -20 mV. In frog atrial muscle if the membrane potential is clamped to values more negative than -50 mV during the period of Ca deprivation, no contracture develops on the restoration of the extracellular Ca concentration ([Ca]o). In all tissues the depolarization, the rise in [Na]i, and the Ca addition contracture are blocked by Ca channel blockers, ant...
The Journal of Physiology, 1978
International Journal of Evolutionary Biology, 2012
We present a comprehensive phylogenetic analysis of the Utaka, an informal taxonomic group of cic... more We present a comprehensive phylogenetic analysis of the Utaka, an informal taxonomic group of cichlid species from Lake Malawi. We analyse both nuclear and mtDNA data from five Utaka species representing two (Copadichromis and Mchenga) of the three genera within Utaka. Within three of the five analysed species we find two very divergent mtDNA lineages. These lineages are widespread and occur sympatrically in conspecific individuals in different areas throughout the lake. In a broader taxonomic context including representatives of the main groups within the Lake Malawi cichlid fauna, we find that one of these lineages clusters within the non-Mbuna mtDNA clade, while the other forms a separate clade stemming from the base of the Malawian cichlid radiation. This second mtDNA lineage was only found in Utaka individuals, mostly within Copadichromis sp. "virginalis kajose" specimens. The nuclear genes analysed, on the other hand, did not show traces of divergence within each species. We suggest that the discrepancy between the mtDNA and the nuclear DNA signatures is best explained by a past hybridisation event by which the mtDNA of another species introgressed into the ancestral Copadichromis sp. "virginalis kajose" gene pool.
International Journal of Evolutionary Biology, 2011
Peripheral isolated populations may undergo rapid divergence from the main population due to vari... more Peripheral isolated populations may undergo rapid divergence from the main population due to various factors such as a bottleneck or a founder effect followed by genetic drift or local selection pressures. Recent populations of two economically important Copadichromis species in Lake Malombe, a satellite lake of Lake Malawi, were neither genetically nor morphometrically distinct from their source populations in the main lake. Evidence was found for a founder effect which had a different impact on the genetic composition of the two species. In addition, the increased fishing pressure in Lake Malombe may have led to a reduction of the body sizes of both species.
Journal of Molecular and Cellular Cardiology, 1986
- 18, [241][242][243][244][245][246][247][248][249][250][251][252][253][254]. The intracellul... more 1986) 18, [241][242][243][244][245][246][247][248][249][250][251][252][253][254]. The intracellular sodium content of cardiac cells in fish and amphibia, measured with either an isotope technique or with sodium-sensitive micro-electrodes, rises steeply from around 15 mmol/l in calciumcontaining solution to as much as 70 mmol/l, during exposure to a Ca 2 +-free solution. This increase is associated with the development of spontaneous and prolonged action potentials so that the membrane may stabilise around --20 mV. On reperfusion with calcium-containing medium the membrane repolarises before a strong contracture develops. Inhibition of the Na-pump increases both the sodium gain and the subsequent calcium re-admission tension. A number of agents e.g. divalent cations, anti-arrhythmic drugs, local anaesthetics and Ca-channel blockers are able to prevent the development of the contracture but only if they are present during the calcium-free perfusion. They also inhibit the development of spontaneous electrical activity and the rise in Na i . The calcium re-admission contracture can be blocked in amphibian preparations voltage clamped around the resting potential during low calcium perfusion. From the known pharmacological action of these agents and the voltage and time dependence of the calcium channel, it is concluded that during calcium depletion, the prolongation of the action potentials is associated with a sustained entry ofNa + via the Ca-channels which leads to the rise in Na i . Once Na i has risen, these agents with the exception of Mn 2+, a known inhibitor of the Na/Ca exchange, are unable to prevent the development of the contracture. This suggests that the re-admission contracture follows calcium uptake by way of the Na/Ca exchange.
Journal of Molecular and Cellular Cardiology, 1991
Journal of Molecular and Cellular Cardiology, 1991
In rat papillary muscle, rapid cooling causes membrane depolarization which initiates action pote... more In rat papillary muscle, rapid cooling causes membrane depolarization which initiates action potentials that lead to a contraction. This rapid cooling contraction (RCC) can be blocked by TTX, Mn2+, Ni2+ or high K+ superfusion. In the presence of caffeine (0.5-1 mM), the rapid cooling contracture (caffeine-RCC) has an amplitude similar to that of a twitch elicited by field stimulation at 37 degrees C, but is not inhibited by these agents. As the caffeine-RCC appears to be independent of membrane depolarization and Ca influx but can be inhibited by increasing the bathing caffeine concentration to 20 mM, we consider that the amplitude of this contracture gives a good indication of the calcium content of the sarcoplasmic reticulum (SR). In Tyrode containing 1.8 mM Ca an increased stimulus frequency leads to a negative force staircase which is paralleled by a similar decrease in the amplitude of the caffeine-RCC. These effects are lost if the bathing Ca is reduced (0.18-0.45 mM) in a way which can be reversed by isoproterenol (100 nM). In verapamil (2 microM), however whilst the twitch responses may show a steeper dependence upon stimulus frequency, the negative frequency dependence of the caffeine-RCC is also lost. Low external Na+ also inhibits the frequency dependent reduction of the caffeine-RCC. The results suggest that if the amplitude of the caffeine-RCC is a good indication of the SR calcium content, then this Ca store is related reciprocally to membrane Ca current where activation of the Ca channels leads to a depletion of the store whereas inhibition of membrane Ca channels leads to a filling of the Ca store. We propose that on stimulation the size of the Ca influx determines the fraction of Ca released from the SR. This released Ca may be partially extruded from the cell by way of the Na/Ca exchange which acts in competition with the re-uptake mechanism of the SR to control SR Ca content.