Muscarinic-receptor induced myo-inositol trisphosphate accumulation, myosin light chain phosphorylation and contraction in the rabbit iris sphincter smooth muscle.

The data presented in this communication are in accord with the hypothesis that a synergistic interaction between IP3 and DG, the two arms of the polyphosphoinositide cycle, could play an important role in smooth muscle contraction (summarized in Fig. 2). Thus: (a) Dose-response relationships between IP3 accumulation, MLC phosphorylation and muscle contraction for CCh suggest a fairly good correlation between the biochemical and pharmacological responses. The finding that the CCh effects were blocked by the muscarinic antagonists, atropine and pirenzepine, suggests that both of these responses are controlled by muscarinic cholinergic receptors. The studies with pirenzepine suggest that IP3 accumulation as well as contraction in this smooth muscle are mediated by low affinity M2 receptors. (b) The kinetic data on atropine and pirenzepine antagonism also suggest a close relationship between IP3 accumulation, MLC phosphorylation and contraction, and furthermore they suggest that both the biochemical and pharmacological responses are competitively inhibited by the muscarinic antagonists. (c) Further evidence for a close relationship between agonist-stimulated PIP2 hydrolysis, MLC phosphorylation and contraction is provided by the time-course studies carried out at two different temperatures, 37 degrees C and 4 degrees C, and in the presence and absence of Ca2+. The data obtained from these studies indicate that the biochemical response may precede the physiological response in this tissue. (d) Finally, the studies on determining whether or not the C-kinase branch of the Ca2+ messenger system might play a role in regulating the tonic phase of agonist-induced smooth muscle contraction revealed that the phorbol ester, PDBu, but not PMA, induced MLC phosphorylation and contraction in a dose and time dependent manner. In addition, when added in combination with ionomycin, PDBu acted in a synergistic manner potentiating both the amount and rapidity of smooth muscle contraction. These findings suggest the presence of a C-kinase mediated contractile mechanism in the iris sphincter. The differential effects of phorbol esters observed in the present work could be due to the possibility that the iris sphincter is permeable to PDBu but not to PMA.(ABSTRACT TRUNCATED AT 400 WORDS)