Effects of Ca2+ and K+ channel blockers on nerve impulses recorded from guinea-pig postganglionic sympathetic nerve terminals (original) (raw)
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British Journal of Pharmacology, 1993
The specific type(s) of voltage-sensitive calcium channels (VSCCs) involved in sympathetic neurotransmission have not yet been characterized in human vascular tissues. We therefore examined the functional role of the N-and L-type VSCCs in human saphenous veins. 2 Contractile response curves for transmural nerve stimulation (TNS) and for exogenously administered noradrenaline (NA) were obtained in superfused saphenous vein rings. The contractions induced by TNS, but not by NA, were inhibited by 1 jLM tetrodotoxin and by 10 ItM guanethidine. Both responses were substantially reduced by 1 jtM phentolamine, indicating that the contractions evoked by TNS were mediated by endogenous NA released from noradrenergic nerves. 3 In the presence of 2 gM w-conotoxin GVIA (omega Conus Geographus toxin, fraction VI A; w-CgTx), a polypeptide with specific inhibitory activity on N-and L-type calcium channels, the neurally evoked contractions were almost completely abolished. In contrast, the responses induced by exogenous NA were not affected by the neurotoxin, thus providing evidence of the exclusive presynaptic action of w-CgTx. 4 In the presence of the calcium antagonist verapamil (10 gM), which selectively blocks L-type VSCCs, the contractions induced by both TNS and NA were diminished to the same extent, suggesting that the organic calcium blocker is active only at the postjunctional level. 5 It is concluded that N-type calcium channels are the main pathway of calcium entry controlling the functional responses induced by activating sympathetic nerves; the role of L-type channels appears to be limited to the postjunctional level, modulating smooth muscle contractions.
Journal of neurophysiology, 2000
The types of Ca(2+)-dependent K(+) channel involved in the prolonged afterhyperpolarization (AHP) in a subgroup of sympathetic neurons have been investigated in guinea pig celiac ganglia in vitro. The conductance underlying the prolonged AHP (gKCa2) was reduced to a variable extent in 100 nM apamin, an antagonist of SK-type Ca(2+)-dependent K(+) channels, and by about 55% in 20 nM iberiotoxin, an antagonist of BK-type Ca(2+)-dependent K(+) channels. The reductions in gKCa2 amplitude by apamin and iberiotoxin were not additive, and a resistant component with an amplitude of nearly 50% of control remained. These data imply that, as well as apamin- and iberiotoxin-sensitive channels, other unknown Ca(2+)-dependent K(+) channels participate in gKCa2. The resistant component of gKCa2 was not abolished by 0.5-10 mM tetraethylammonium, 1 mM 4-aminopyridine, or 5 mM glibenclamide. We also investigated which voltage-gated channels admitted Ca(2+) for the generation of gKCa2. Blockade of Ca(2...
Role of Ca 2+Activated K + Channels on Adrenergic Responses of Human Saphenous Vein
American Journal of Hypertension, 2007
We studied the participation of K+ channels on the adrenergic responses in human saphenous veins as well as the intervention of dihydropyridine-sensitive Ca2+ channels on modulation of adrenergic responses by K+ channels blockade.Saphenous vein rings were obtained from 40 patients undergoing coronary artery bypass surgery. The vein rings were suspended in organ bath chambers for isometric recording of tension.Iberiotoxin (10−7 mol/L), an inhibitor of large conductance Ca2+-activated K+ channels, and charybdotoxin (10−7 mol/L), an inhibitor of both large and intermediate conductance Ca2+-activated K+ channels, enhanced the contractions elicited by electrical field stimulation and produced a leftward shift of the concentration–response curve to norepinephrine. In contrast, the inhibitor of small conductance Ca2+-activated K+ channels apamin (10−6 mol/L) did not modify the contractile response to electrical field stimulation or norepinephrine. In the presence of the dihydropyridine Ca2+-channel blocker nifedipine (10−6 mol/L), iberiotoxin and charybdotoxin failed to enhance the contractile responses to electrical field stimulation and norepinephrine.The results suggest that large conductance Ca2+-activated K+ channels are activated by stimulation with norepinephrine to counteract the adrenergic-induced contractions of human saphenous vein. Thus, inhibition of these channels increases significantly the contraction, an effect that appears to be mediated by an increase in Ca2+ entry through L-type voltage-dependent Ca2+ channels.
British Journal of Pharmacology, 1997
1. The effects of the voltage-dependent calcium channel (VDCC) blockers omega-agatoxin IVA (omega-AgaIVA), omega-conotoxin GVIA (omega-CgTx), omega-conotoxin MVIIC (omega-MVIIC) and omega-conotoxin MVIID (omega-MVIID) were evaluated on transmitter release in the mouse diaphragm preparation. The effects of omega-AgaIVA and omega-MVIIC were also evaluated on the perineurial calcium and calcium-dependent potassium currents, ICa and IK(Ca), respectively, in the mouse levator auris preparation. 2. The P- and Q-type VDCC blocker omega-AgaIVA (100 nM) and P- Q- and N-type channel blockers omega-MVIIC (1 microM) and omega-MVIID (3 microM) strongly reduced transmitter release (> 80-90% blockade) whereas the selective N-type channel blocker omega-CgTx (5 microM) was ineffective. 3. The process of release was much more sensitive to omega-MVIIC (IC50 = 39 nM) than to omega-MVIID (IC50 = 1.4 microM). After almost completely blocking transmitter release (quantal content approximately 0.3% of its control value) with 3 microM omega-MVIIC, elevating the external [Ca2+] from 2 to 10 mM induced an increase of approximately 20 fold on the quantal content of the endplate potential (e.p.p.) (from 0.2 +/- 0.04 to 4.8 +/- 1.4). 4. Nerve-evoked transmitter release in a low Ca(2+)-high Mg2+ medium (low release probability, quantal content = 2 +/- 0.1) had the same sensitivity to omega-AgaIVA (IC50 = 16.8 nM) as that in normal saline solutions. In addition, K(+)-evoked transmitter release was also highly sensitive to the action of this toxin (IC50 = 11.5 nM; 100 nM > 95% blockade). The action of omega-AgaIVA on transmitter release could be reversed by toxin washout if the experiments were carried out at 31-33 degrees C. Conversely, the effect of omega-AgaIVA persisted even after two hours of toxin washout at room temperature. 5. Both the calcium and calcium-dependent potassium presynaptic currents, ICa and IK(Ca), respectively, were highly sensitive to low concentrations (10-30 nM) of omega-AgaIVA. The ICa and the IK(Ca) were also strongly reduced by 1 microM omega-MVIIC. The most marked difference between the action of these two toxins was the long incubation times required to achieve maximal effects with omega-MVIIC. 6. In summary these results provide more evidence that synaptic transmission at the mammalian neuromuscular junction is mediated by Ca2+ entry through P- and/or Q-type calcium channels.
Calcium-dependent chloride current induced by axotomy in rat sympathetic neurons.
The Journal of physiology, 1994
1. Seven to ten days after sectioning their axons, rat sympathetic neurons were studied using intracellular recording techniques in an in vitro preparation of the superior cervical ganglion. 2. In 75 % of axotomized cells, an after-depolarization (ADP) was observed following spike firing or depolarization with intracellular current pulses. Discontinuous single-electrode voltage-clamp techniques were employed to study the ADP. When the membrane potential was clamped at the resting level just after an action potential, a slow inward current wa.s recorded in cells that showed an ADP. 3. In the presence of ITX and TEA, inward peaks and outward currents were recorded during depolarizing voltage jumps, followed by slowly decaying inward tail currents accompanied by large increases in membrane conductance. The inward peak and tail currents activated between -10 and -20 mV and reached maximum amplitudes around 0 mV. With depolarizing jumps to between +40 and +50 mV, net outward currents were recorded during the depolarizing jumps but inward tail currents were still activated. 4. In the presence of the Ca2+ channel blocker cadmium, or when Ca2+ was substituted by Mg2+, the ADP disappeared. In voltage-clamped cells, cadmium blocked the inward tail currents. The reversal potential for the inward tail current was approximately -15 mV.
The Journal of Physiology, 1995
The effects of the calcium channel blockers, funnel-web spider toxin (FTX), w-agatoxin IVA (w-Aga IVA) and w-conotoxin GVIA (w-CgTX), were tested on transmitter release and presynaptic currents in frog motor nerve endings. 2. Evoked transmitter release was blocked by FTX (IC50= 0o02 #l ml-') and w-CgTX (1 #M) but was not affected by w-Aga IVA (0'5/M). When FTX (0 1 #l ml-') was assayed on spontaneous release either in normal Ringer solution or in low Ca2+-high Mg2+ solution, it was found not to affect miniature endplate potential (MEPP) amplitude but to increase MEPP frequency by-2-fold in both conditions. 3. Presynaptic calcium currents (Ica), measured by the perineurial technique in the presence of 10 mm tetraethylammonium chloride (TEA) and 200 juM BaCl2 to block K+ currents, were blocked by w-CgTX (5 ,UM), partially blocked by FTX (1 1d ml-') and not affected by w-Aga IVA (0 5,UM). 4. The presynaptic calcium-activated potassium current (IK(ca)) measured by the perineurial technique in the presence of 0 F5jM 3,4-aminopyridine (DAP) to block voltage-dependent K+ currents, was strongly affected by charybdotoxin (ChTX) (300 nM) and completely abolished by BaCl2 (200 juM). This current was also blocked by w-CgTX (5 FM) and by CdC12 (200 FM) but was not affected by FTX (1 Fl ml-'). The blockade by w-CgTX could not be reversed by elevating [Ca]o to 10 mM. 5. The results suggest that in frog synaptic terminals two w-CgTX-sensitive populations might coexist. The transmitter release process seems to be mediated by calcium influx through a w-CgTXand FTX-sensitive population.