Abstract
In smooth muscle cells isolated from swine renal interlobar arteries, phenylephrine (PE) at concentrations of 1-10 microM produced biphasic increases of the intracellular calcium concentration. An early transient rise was followed by a maintained plateau. The maintained component was sensitive to extracellular calcium, in contrast to the early transient, which was still observed in nominally calcium-free solution. Nifedipine (1 microM) and NiCl2 (100 microM) only weakly affected the calcium signal, suggesting that voltage-sensitive calcium channels play only a minor role in the PE-induced changes in intracellular calcium. Thapsigargin (0.5 microM) elevated the intracellular calcium concentration and depressed both the early transient and the maintained component of the PE response. In calciumfree medium PE induced a transient rise of the intracellular calcium concentration with a depressed plateau. Readmission of calcium elevated the intracellular calcium concentration above the baseline. Both components of the PE-induced calcium signal were completely abolished when the cells were pretreated with the phospholipase C (PLC) inhibitor U73122 (2 microM). LaCl3 (100 microM, 1 mM), an inhibitor of calcium-release-activated current (ICRAC), had no effect on the PE-induced calcium signal. GdCl3 (50 microM), SKF 96365 (10 microM) and flufenamic acid (100 microM), reported to inhibit nonselective cation channels, blocked or transiently reduced the maintained calcium signal. Several protein kinase inhibitors such as genistein (10 microM), H7 (50 microM), H89 (1 microM) and bisindolylmaleimide (0.2 microM) reduced the maintained calcium signal. We conclude that the initial transient spike of the PE-induced calcium signal is due to release of calcium from inositol 1,4,5-trisphosphate-sensitive calcium stores evoked by alpha 1-adrenoceptor-coupled stimulation of PLC and that the maintained component is due to capacitative calcium entry, which is modulated by protein kinases.
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