Modulation of Aortic Protein Phosphatase Activity by Polylysine

Abstract

The Ca²⁺-regulatory mechanism for contraction of smooth muscle involves phosphorylation of the 20,000-Da myosin light chains. In the fully relaxed muscle the intracellular concentration of Ca²⁺ is low (<10^-9 M) and no interaction between the contractile proteins, actin and myosin, is evident. Following excitation, however, release of Ca²⁺ from intracellular storage sites and influx of Ca²⁺ from the extracellular space through voltage-dependent and receptor-operated channels in the sarcolemma results in a marked increase in the intracellular concentration of Ca²⁺ (>10^-6 M). Subsequently, formation of the Ca²⁺-calmodulin complex results in activation of myosin light-chain kinase and phosphorylation of the regulatory light chains. In this form, actin-myosin interaction is activated resulting in hydrolysis of ATP and cross-bridge cycling. The physiological correlate of these biochemical events is contraction of the smooth muscle cell (see (1–4) for reviews). Although recent studies suggest that cross bridges may remain attached after dephosphorylation of the light chains (5), cyclic phosphorylation and dephosphorylation appear to be requisite for recurring contraction and relaxation of smooth muscle.

Phosphorylase, the rate-limiting enzyme in glycogenosis, is also phosphorylated during contraction of a variety of smooth muscles (6–9). Moreover, recent studies suggest that glycogenosis may participate in generating ATP required for contraction of vascular smooth muscle (10). Accordingly, phosphatases which are effective in dephosphorylating phosphorylase a and the myosin light chains may function in coordinating metabolism and contractility in smooth muscle (11–13).

Smooth Muscle Phosphatases. Gizzard. At least four phosphatases, designated SMPI-IV, have been identified in the smooth muscle of turkey gizzard (14–17). Two of these have been purified to apparent homogeneity: SMPI consists of three subunits (M _r = 65,000; 50,000; 38,000) and SMPII is a Mg²⁺-dependent monomelic enzyme (M _r = 43,000) which appears to be closely related to a phosphatase isolated from rat liver (18) and bovine myocardium (19).