Effect of staurosporine on the phorbol ester-induced activation of the Na+/H+ antiporter in smooth muscle cells

Modulation of the Na-H antiport by insulin: interplay between protein kinase C, tyrosine kinase, and protein phosphatases

The insulin modulation of Na-H antiport in rat hepatocytes was studied using the fluorescent, pH-sensitive intracellular probe, 2',7' bis (carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Our data show that insulin stimulates the Na-H antiport. The dose-response of insulin effect shows a behavior typical of other insulin responses: a maximum in the physiological range (1 nM) and smaller effects at higher and lower hormone concentrations. The time-course of activation is very fast at high hormone concentrations and slow, but reaching a higher value, for the physiological concentrations (0.26 +/- 0.05 and 0.18 +/- 0.022 pH units for 1 nM and 1 microM insulin respectively). The use of phorbol, 12-myristate, 13-acetate (PMA), a potent activator of protein kinase C and its inhibitor staurosporine, and the inhibitor of tyrosine kinase erbstatin analog, suggests that both protein kinase C and tyrosine kinase could be involved in the mechanism leading to Na-H antiport activation by insulin. We suggest that the activation of the antiport involves the two pathways depending on the hormone concentration. In particular, protein kinase C would mediate the effects of high hormone concentrations, acting as a growth factor, since staurosporine fully inhibited insulin 1 microM, but only partially 1 nM effects, and tyrosine kinase would mediate the effect of insulin 1 nM and only partially 1 microM. Okadaic acid 1 microM, a potent inhibitor of protein phosphatases, mimicked the hormone effects on the antiport and abolished the different time-course due to hormone concentration, suggesting a role of kinases and phosphatases in the signal transduction. The effect of all activators was abolished by amiloride analog, 5-(N-ethyl-N-isopropyl) amiloride (EIPA), confirming the specificity of these effects.

Effects of a phorbol ester and staurosporine on electro- and pharmacomechanical coupling in a resistance artery

We examined the role of protein kinase-C in contractile responses of small arteries of the rat by stimulating and inhibiting protein kinase-C with phorbol myristate acetate and staurosporine, respectively. The experiments were performed in isolated mesenteric resistance arteries that had been sympathectomized and mounted for recording of isometric force development. Phorbol myristate acetate (i) at concentrations lower than 3 nM increased sensitivity for the contractile effect of potassium, but not for the effect of noradrenaline or BAY-K8644, (ii) at concentrations higher than 30 nM increased the sensitivity of depolarized vessels to extracellular calcium and (iii) at concentrations higher than 30 nM induced a contractile effect that depended on the presence of extracellular calcium and that was reduced by the calcium antagonist felodipine. Neither the phorbol ester nor staurosporine affected contractile responses to caffeine in calcium-free solution. Staurosporine (10 nM) reduced the response of resistance arteries to potassium but not to noradrenaline. These results are in agreement with direct observations by others that protein kinase-C plays a role in the activation of voltage-operated calcium channels. Protein kinase-C could participate in this way in electro-mechanical coupling in resistance arterial smooth muscle and, when strongly activated, sensitize the contractile apparatus to calcium.