Intracellular calcium levels in phorbol ester-induced contractions of vascular muscle

Abstract

Intracellular calcium concentration ([Ca2+]i) was measured with aequorin in ferret and rat aortic strips contracted with phorbol esters. In ferret aorta, 12-deoxyphorbol 13-isobutyrate 20-acetate (DPBA, 1 microM) induced contractions without significantly increasing [Ca2+]i, whereas 21 mM K+ induced smaller contractions with a significant rise in [Ca2+]i. Ca2+-free 2.5 mM ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA)-physiological saline solution (PSS) had no effect on DPBA-induced tension, whereas it abolished contractions induced by 66 mM K+. The alpha 1-adrenergic agonist phenylephrine (10(-5) M) induced less than 10% of the tension with no initial [Ca2+]i spike under Ca-free conditions. In rat aorta, both phorbol 12-myristate 13-acetate (PMA, 2 microM) and DPBA (1 microM) induced contractions without increasing [Ca2+]i; Ca2+-free EGTA-PSS or the addition of the calcium channel blocker gallopamil (D600, 1 microM), however, abolished greater than 50% of the tension induced by either phorbol ester with a decrease in [Ca2+]i. These results are consistent with the idea that 1) resting [Ca2+]i is both sufficient and required to support phorbol ester-induced contractions in two vascular smooth muscles, suggesting an increased sensitivity of the contractile apparatus for Ca2+, and 2) there are differences in the mechanisms by which phorbol esters and alpha 1-agonists may activate vascular smooth muscle.

Effects of flavonoids on rat aortic smooth muscle contractility: Structure-activity relationships

“…Protein kinase C (PKC) has been proposed to play a key role in maintenance of tonic contractions of vascular smooth muscle (Rasmussen et al, 1987). Phorbol esters (i.e., PMA) activate PKC and induce a slowly developing sustained contraction in rat aorta without changing [Ca]i, possibly by increasing the Ca 2+ sensitivity of contractile proteins (Jiang and Morgan, 1987;Karaki, 1989;Duarte et al, 1994). PKC from rat brain was inhibited by plant flavonoids in a concentrationdependent manner depending on the flavonoid structure (Ferriola et al, 1989).…”

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

“…10 On the other hand, Galizzi et al" have observed that preexposure of A7r5 cells to phorbol esters for 10 minutes inhibits KC1 or Bay K8644-stimulated 45 Ca uptake. Other studies using aequorin as [Ca 2+ ]j indicator, however, have shown that phorbol esters do not significantly increase [Ca 2+ ]j in VSM strips 12 or tracheal smooth muscle strips. 13 To assess the changes in [Ca 2+ ]j during hormonal stimulation and their regulation by A-and Ckinases, we have measured [Ca 2+ ]j in individual human umbilical vein cells before and after hormonal stimulation.…”

Regulation of contraction and relaxation in arterial smooth muscle.

“…- 63 In the swine carotid, we found that low dose (10 nM) phorbol dibutyrate, a water soluble phorbol diester, produced very small but significant increases in [Ca 2+ ]i and phosphorylation, yielding a [Ca 2+ ]| sensitivity of phosphorylation similar to that induced by histamine. 64 ]i sensitivity of phosphorylation.…”