Pneumadin-evoked intracellular free Ca 2+ responses in rat aortic smooth muscle cells: effect of dexamethasone

Abstract

The direct vascular effect of pneumadin (PN) was determined by studying the changes in intracellular free calcium ([Ca 2+] i) levels in cultured rat aortic smooth muscle cells maintained between the second and fifth passages. PN evoked a rapid, concentration-dependent, biphasic increase in [Ca 2+] i. The [Ca 2+] i level rose from a basal value of 108 nM to a maximum increase in peak value of 170 nM. Although the level of maximal [Ca 2+] i response evoked by PN was less than with other vasoactive agonists, it was more potent ( ec 50 0.5 nM) than even endothelin-1 ( ec 50 3.1 nM). At concentrations > 100 nM, [Ca 2+] i elevations induced by PN above basal levels were no longer observed. Pretreatment with dexamethasone (100 nM for 24 hr) resulted in a significant increase ( P < 0.01) in the peak [Ca 2+] i response (310 nM) to PN. However, the biphasic pattern in the peak [Ca 2+] i responses encountered with increasing concentrations of PN remained unaffected. The exaggerated [Ca 2+] i response to PN was abolished by preincubation of cells with either the glucocorticoid antagonist mifepristone (RU 486) or the protein synthesis inhibitor cycloheximide. Inclusion of either an AT 1 antagonist (losartan), a V 1 selective vasopressin antagonist (d(Ch 2) 5 Tyr (Me) AVP), or an α-adrenoceptor antagonist (phentolamine) failed to affect the increases in [Ca 2+] i induced by PN. PN-evoked increases in inositol 1,4,5-trisphosphate levels paralleled the [Ca 2+] i changes. These data suggest that PN increases Ca 2+ mobilization in rat aortic smooth muscle cells via activation of phospholipase C coupled receptors. This effect is up-regulated by dexamethasone.