Cytosolic calcium levels in vascular smooth muscle.

Increase in cytosolic Ca2+ level ([Ca2+] cyt) is prerequisite for smooth muscle contraction. Simultaneous measurements of [Ca2+] cyt and muscle tension give direct information for the Ca2(+)-regulation of smooth muscle. A fluorescent Ca2+ indicator, fura-2, is used for this purpose. Comparison between [Ca2+] cyt and muscle tension in vascular smooth muscle indicates that, although high K+ and receptor-agonists such as norepinephrine and prostaglandin F2 alpha induce sustained contraction by the sustained increase in [Ca2+] cyt, greater contraction is produced by receptor-agonists than high K+ at a given [Ca2+]cyt. Phorbol ester show similar effects as receptor-agonists, and it potentiates a high K(+)-induced contraction with little effect on [Ca2+]cyt. These results suggest that the contraction of smooth muscle is due to the increase in [Ca2+]cyt. Furthermore, receptor-agonists stimulate phosphatidylinositol turnover and generates diacyl glycerol which activates protein kinase C and may consequently increase the Ca2+ sensitivity of contractile elements. The [Ca2+]cyt -dependent portion of these contractions is inhibited by Ca2+ channel blockers such as verapamil by the decrease in [Ca2+]cyt. By contrast, increase in cyclic AMP by isoproterenol and forskolin inhibits smooth muscle contraction by the decrease in [Ca2+]cyt also by the decrease in the Ca2+ sensitivity of contractile elements. Increase in the cyclic GMP level by sodium nitroprusside show effects quite similar to those of cyclic AMP. Thus, contractility of vascular smooth muscle seems to be regulated by [Ca2+]cyt and also by Ca2+ sensitivity of the contractile elements. Furthermore, at least part of the receptor-mediated changes may be due to activation of protein kinase C.