Signal transduction in smooth muscle - Selected contribution: HSP20 phosphorylation in nitroglycerin- and forskolin-induced sustained reductions in swine carotid media tone

Cyclic nucleotide-induced relaxation of maximally activated arterial smooth muscle has two phases. 1) The initial relaxation transient is typically ch aracterized by a rapid reduction in force associated with brief reductions in myoplasmic Ca2+ concentration ([Ca2+](i)) and myosin regulatory light ch ain (MRLC) phosphorylation on serine (Ser)-19 (Ser(19)). 2) The sustained i nhibitory response is typically associated with Ser(16) phosphorylation of heat shock protein 20 (HSP20) without sustained reductions in [Ca2+](i) or MRLC phosphorylation. We investigated whether the extent of Ser(16)-HSP20 p hosphorylation quantitatively correlated with the sustained inhibitory resp onse. With addition of nitroglycerin to histamine-stimulated swine carotid media, the initial relaxation transient was associated with a decrease in M RLC phosphorylation without an increase in Ser(16)-HSP20 phosphorylation. D uring the sustained phase of nitroglycerin-induced relaxation and during fo rce redevelopment induced by washout of nitroglycerin in the continued pres ence of histamine, the level of Ser(16)-HSP20 phosphorylation, but not MRLC phosphorylation, correlated with inhibition of force. Forskolin, which inc reases cAMP concentration, also induced a sustained inhibitory response tha t was associated with increases in Ser(16)-HSP20 phosphorylation without re ductions in MRLC phosphorylation levels. Forskolin increased Ser(16)-HSP20 phosphorylation to a greater extent and inhibited force more completely tha n that observed with nitroglycerin. Increases in Ser(16)-HSP20 phosphorylat ion correlated with the degree of force inhibition regardless of whether th e relaxation was induced by nitroglycerin or forskolin. These data are cons istent with the hypothesis that Ser(16)-HSP20 phosphorylation may be a cycl ic nucleotide-dependent, yet MRLC phosphorylation-independent, inhibitor of smooth muscle contractile force.