Carbachol stimulates adenylate cyclase and phospholipase C and muscle contraction-relaxation in a reciprocal manner in dog iris sphincter smooth muscle (original) (raw)

1992, European Journal of Pharmacology Molecular Pharmacology Section

In the dog iris sphincter, muscarinic acetylcholine receptors are coupled either to the stimulation of phospholipase C and muscle contraction or to the stimulation of adenylate cyclase and muscle relaxation, this was found to be dependent upon the concentration of the muscar]nic agonist. In contrast to the dog, muscarinic receptors in iris sphincters from different mamtp.alian species were found t,9 be coupled to phospholipase C and contraction at all concentrations of earbachol investigated (1-100/z M). In the dog sphincter, lower concentrations (< 5 #M) of carbachol stimulated myo-inositol 1,4,5-trisphosphate (IP0 production, inhibited cAMP formation and induced contraction, and higher concentrations (> 5/aM) enhanced cAMP formation, inhibited IP 3 production and induced re!a×ation. The mechanisms for the stimulatory effects on cAMP formation through muscarinic receptors were investigated. Carbachol (25 tJ.M) increased both basal and isoprotel'enot-and forskoiit~-stin~ulated cAMP levels. Atropine inhibited the carbachol-stimulated increase in cAMP levels in a dose-dependent manner with an IC5( ~ of 9 nM. lntraceilular Ca"+, derived from !P3-induced Ca -'+ release and/or from muscarinic receptor-operated Ca 2+ influx, and protein kinase C may mediate the muscarinic receptor-linked rise in intracellular cAMP. This conclusion is supported by the following findings. (1) At short time intervals ( < 1 min) carbacho! (25 t~M) increased IP 3 production and contraction and this was followed (between 1 and 20 min) by cAMP formation and muscle relaxation. (2) Carbachol-stimulated 1P 3 production was detected at a concentration of the agonist 26-fold lower than that required for cAMP formation, and it was completely blocked by the phorbot ester, phorbol 12,13-dibutyrate (50 nM). (3) A Ca2+-calmodulin stimulated adenylate cyclase was demonstrated in mcmb~anes fr~,m dog iris sphincter but not in that from rabbit and bovine. (,~) Trifluoperazine (C.I ~M)~ a calmodulin antagonist, inhibited the carbachol-stimulated cAMP accumulation. (51 The Ca z+ ionophore ATq187 ~tnd the phorbol ester increased cAM):' production in a dose-dependent manner. A23187 potentiated cAMP production induced by either carb~cho[ or by 'he phnrbol ester. (6) Muscarinic stimulation of cAMP production persisted even after the tissue was pretreated with the phorbol e~,:er or staurosporine. (7) Nifedipine (0.01-0.5/aM), a Ca 2+ channel antagonist, inhibited carbachol stimulation of cAMP pr(~daction, suggesting the presence of a muscarinic receptor-operated Ca z+ influx pathway in this tissue. The above data are interesting in three ways: (a) They demonstrate an important species difference in the second messenger systems coupled to the activation of muscarin:c receptors, (.O) they demonstrate a reciprocal re!ationsifip (c~o~s-ta|k) bctwcc~ cAMP af~d IP~ pl~oduction, and (c) they suggest that both intracellular and cxtracellular Ca -'+ mobilization may be involved in muscarinic sti~nulation of cAMP production. We suggest that in dog iris sphincter a rise in intraeeltular cAMP levels by cholinergic stimulation is needed to regulate muscarinic receptor-linked Ca -'+ mobilization and mu~le contraction-relaxation. Cyclic AMP production through muscarinic activation could also be required for the functioning of Ca '~ channels in this excitable tissue.