Cyclic Guanosine Monophosphate-Enhanced Sequestration of Ca by Sarcoplasmic Reticulum in Vascular Smooth Muscle

The purpose of this study was to investigate the effects of the intracellular messenger cyclic GMP (cGMP) on sequestration of cytosolic calcium (Ca) into the intracellular Ca 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 "Ca uptake in monolayers of cells. The intracellular store was loaded with Ca by exposing the skinned cells to a "Ca-labeled l-|xM free Ca-containing solution for varying durations (0-20 minutes). Addition of 10 p.M cGMP to six monolayers increased both the initial Ca uptake at 2 minutes (control, 240 ±8 pmol Ca'VlO* 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 Ca/10* cells; +cGMP 1.12±0.03, p<0.02). This stimulation of uptake was quantitatively similar to that caused by 10 JJLM cyclic AMP. It occurred at varying ambient cytosolic Ca concentrations (0.1-1.0 \iM Ca) and was not further enhanced by addition of 10 \LM cGMP-dependent protein kinase. The dose-response of stimulation of Ca uptake with cGMP indicated an EDS of 5 nM cGMP. The release of Ca 2+ from the sarcoplasmic reticulum in response to inositol 1,4,5-trisphosphate 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 Ca by the sarcoplasmic reticulum. (Circulation Research 1988;62:961-964)