Effects of the Ca agonist Bay K8644 on 45Ca influx and net Ca uptake into rabbit aortic smooth muscle

We examined the effects of Bay K8644 and external K+ on isolated rings of rabbit aorta. Previous evidence has suggested that the Ca channel agonist, Bay K8644, preferentially opens the potential-sensitive Ca2+ channels (PSCs) rather than receptor-operated Ca2+ channels (ROCs). Bay K8644 stimulated 45Ca influx, net Ca gain and contractile tension were measured as a function of extracellular K+ concentration. Although Bay K8644 induced little tension at 5 mM K+ (PSS), Ca2+ influx was stimulated from a resting value of 12 mumol X kg-1 X min-1 (0.02 pmol X cm-2 X s-1) to 17 mumol X kg-1 X min-1 while the total Ca content increased by 95 mumol X kg-1. This net Ca gain was reversed by 50 mM caffeine. As the external K+ was increased from 5 to 80 mM, the Bay K8644 stimulation of influx and contraction first increased and then decreased. The data are explained in terms of a calcium entry model consisting of membrane Ca2+ channels in series with a subplasmalemmal barrier consisting of superficial sarcoplasmic reticulum (SR).