Cyclic guanosine monophosphate-enhanced sequestration of Ca2+ by sarcoplasmic reticulum in vascular smooth muscle

Abstract

The purpose of this study was to investigate the effects of the intracellular messenger cyclic GMP (cGMP) on sequestration of cytosolic calcium (Ca2+) into the intracellular Ca2+ store (the sarcoplasmic reticulum) of vascular smooth muscle. Using saponin-skinned primary cultures of rat aortic smooth muscle, we investigated the effect of cGMP on 45Ca uptake in monolayers of cells. The intracellular store was loaded with Ca2+ by exposing the skinned cells to a 45Ca-labeled 1-μM free Ca2+-containing solution for varying durations (0-20 minutes). Addition of 10 μM cGMP to six monolayers increased both the initial Ca2+ uptake at 2 minutes (control, 240 ± 8 pmol Ca2+/106 cells; + cGMP 295 ± 7; mean ± SEM; n - 6, p < 0.01) and the final steady-state uptake reached at 20 minutes (control, 0.96 ± 0.03 nmol Ca2+/106 cells; +cGMP 1.12 ± 0.03, p < 0.02). This stimulation of uptake was quantitatively similar to that caused by 10 μM cyclic AMP. It occurred at varying ambient cytosolic Ca2+ concentrations (0.1-1.0 μM Ca2+) and was not further enhanced by addition of 10 μM cGMP-dependent protein kinase. The dose-response of stimulation of Ca2+ uptake with cGMP indicated an ED50 of 5 nM cGMP. The release of Ca2+ from the sarcoplasmic reticulum in response to inositol 1,4,5-triphosphate or caffeine was unaffected by cGMP. We conclude that the relaxation of vascular smooth muscle with cGMP-producing vasodilators is mediated in part by sequestration of cytosolic Ca2+ by the sarcoplasmic reticulum.