Correlative studies on the effect of carbachol on myo-inositol trisphosphate accumulation, myosin light chain phosphorylation and contraction in sphincter smooth muscle of rabbit iris.

Previously, we have reported that activation of muscarinic cholinergic receptors in the iris smooth muscle results in a rapid breakdown of phosphatidylinositol 4,5-bisphosphate (PIP2) into 1,2-diacylglycerol and myo-inositol trisphosphate (IP3) and that the stimulated hydrolysis of this phospholipid correlates well with contraction. To determine whether or not there is a causal relationship between PIP2 breakdown and contraction, we have conducted correlative studies on the effects of carbachol (CCh) on PIP2 breakdown, measured as IP3 accumulation, myosin light chain (MLC) phosphorylation and contraction in the rabbit iris sphincter. We have also investigated the effects of time, temperature, atropine antagonism, Ca++ and C-kinase activators on the three measured responses. The data obtained can be summarized as follows: dose-response studies for IP3 accumulation, MLC phosphorylation and contraction revealed a close correlation between these responses; kinetic data on atropine antagonism showed that the three measured responses are competitively inhibited by the muscarinic antagonist; time course studies conducted at low temperature showed that the CCh-induced IP3 accumulation and MLC phosphorylation may precede contraction; time course studies on the effect of Ca++ on the three measured responses showed that IP3 release may account for the rapid phase of CCh-induced contraction and that extracellular Ca++ is essential for sustained MLC phosphorylation and the slow phase of contraction; the activity of phospholipase C, the enzyme involved in PIP2 hydrolysis, in membrane fragments from 32P-labeled sphincter muscle was found to be highly sensitive to Ca++, with half-maximal stimulation at about 1.1 microM Ca++; and phorbol 12,13-dibutyrate, but not phorbol 12-myristate 13-acetate, induced MLC phosphorylation and muscle contraction in a dose- and time-dependent manner. Phorbol 12,13-dibutyrate and ionomycin acted in a synergistic manner to elicit contraction. In conclusion, contractions by CCh in the iris sphincter may be explained on the basis of enhanced PIP2 turnover and its derived second messenger molecule(s); that there are consistent correlations, using different concentrations of CCh, atropine antagonism, time, temperature and Ca++, between the stimulated hydrolysis of PIP2, MLC phosphorylation and contraction. Finally, whereas the data presented favor the involvement of IP3 in the phasic component of the contractile response, the studies with phorbol 12,13-dibutyrate suggest that contractile regulation by 1,2-diacylglycerol, through activation of C-kinase, may be important during the tonic component of smooth muscle contraction.