Regulation of cytosolic free Ca2+ concentration in vascular smooth muscle cells by A- and C-kinases.

The cytosolic free Ca2+ concentration [( Ca2+]i) was measured in cultured human umbilical vein smooth muscle cells using fura-2 as a Ca2+ indicator and microscopic digital image analysis system. Activation of cells with histamine and vasopressin resulted in a prompt though transient rise in [Ca2+]i 10- to 12-fold higher than the resting [Ca2+]i. The [Ca2+]i then declined rapidly during the first 30-40 seconds after hormonal stimulation and then gradually decreased to near resting levels in 2-3 minutes in the continued presence of hormones. The magnitude of the increase in peak [Ca2+]i was similar in buffered salt solution containing 1.8 mM Ca2+, zero Ca2+, and zero Ca2+ buffered salt solution containing 10 mM La3+, suggesting that receptor-mediated increase in [Ca2+]i is primarily due to the release of Ca2+ from the intracellular stores. Addition of La3+ produced oscillations in [Ca2+]i in approximately half the cells in response to both hormones. Addition of 10 microM forskolin did not significantly affect the resting [Ca2+]i, the hormone-stimulated peak [Ca2+]i, or the time course of hormone-stimulated [Ca2+]i transients. These data suggest that mechanisms involved in A-kinase-mediated smooth muscle relaxation may be subsequent to the changes in [Ca2+]i. Activation of C-kinase by 1 microM 12 deoxyphorbol 13-isobutyrate-20 acetate (DPBA) did not affect the resting [Ca2+]i, though it attenuated the histamine and vasopressin-mediated peak elevation in [Ca2+]i. Since DPBA inhibited the peak [Ca2+]i response to both the hormones to the same extent, it would appear that C-kinase activation may uncouple the receptor-mediated activation of phospholipase C.