Polyphosphoinositides and Muscarinic Cholinergic and α₁-Adrenergic Receptors in the Iris Smooth Muscle

Summary

An attempt was made in this brief review first to recount some of our early studies which culminated in the characterization of the polyphosphoinositide (phosphatidylinositol 4-monophosphate, PIP and phosphatidylinositol 4,5-bisphosphate, PIP₂) effect in the iris smooth muscle, and second to present more recent data on the rapid breakdown of ³² P-prelabeled polyphosphoinositide (PPI) and release of [³H]myo-inositol phosphates by carbachol (CCh) in this tissue. The PPI effect is defined as the agonist-stimulated breakdown of PIP₂ into diacylglycerol, measured as labeled phosphatidate, and inositol trisphosphate (IP₃). These early findings included: (1) the demonstration of an agonist-stimulated breakdown of PIP₂ which occurred at relatively short time intervals (2.5–10 min), when compared to the phosphatidylinositol (PI) effect reported in a variety of tissues; (2) the demonstration of the PPI effect in vivo, in response to electrical stimulation of the sympathetic nerve of the eye; (3) the demonstration, through pharmacologic and adrenergic denervation supersensitivity studies that PIP₂ breakdown is linked to muscarinic cholinergic and aladrenergic receptors; (4) the demonstration that phosphodiesteratic cleavage of PIP₂ into diacylglycerol and I P₃, by PIP₂ phosphodiesterase, is the molecular mechanism underlying the PPI effect; (5) the demonstration of some requirement for $C{{a}^{{{{2}^{ + }}}}}$, derived mainly from studies on the inhibitory effects of EGTA and $C{{a}^{{{{2}^{ + }}}}}$ ionophore A23187 on this phenomenon; however, the recent finding that the cationophore-stimulated breakdown of PIP₂ is blocked by prazosin leads us now to conclude that while the PPI effect in the iris needs some $C{{a}^{{{{2}^{ + }}}}}$, it is not regulated by intracellular $C{{a}^{{{{2}^{ + }}}}}$; (6) the demonstration of a close correlation between agonist-stimulated PIP₂ breakdown and agonist-induced muscle contraction, which led us to suggest that the agonist-stimulated PIP₂ breakdown is an early event in the pathway which leads from receptor activation to muscle response. Data are also presented which demonstrate that in the iris, the breakdown of labeled PIP₂ and release of IP₃ by CCh occur within 15 s; in contrast the release of IP occurred at longer time intervals (>1 min). Thus after incubation for 15 s with CCh there was 48% loss of ³²P radioactivity from PIP₂ in ³²P-labeled iris, 81% increase in IP₃ release and no change in the release of IP in iris prelabeled with [³H]inositol. These data suggest that agonist-stimulated PIP₂ breakdown is probably involved in the mechanism of both the phasic (fast) and tonic (slow) components of the contractile response. Neither 2-deoxyglucose nor Li⁺, when added for short time intervals (10 min), had any influence on the PPI effect. In accord with our previous studies we conclude that the phosphodiesteratic cleavage of PIP₂ is an early (initial) event in the pathway which leads from activation of $C{{a}^{{{{2}^{ + }}}}}$-mobilizing receptors to muscle response.