Distinct pathways of Ca2+ sensitization in porcine coronary artery - Effects of Rho-related kinase and protein kinase C inhibition on force and intracellular Ca2+

Alterations of the Ca2+ sensitivity of contraction have been reported for p orcine coronary artery, but the mechanisms are not. clearly understood. We investigated the mechanism(s) of Ca2+ sensitization in response to the thro mboxane A, analogue (U46619), Our hypothesis is that different mechanisms o f Ca2+ sensitization could be distinguished by their distinct time courses. Therefore, we measured the time course of [Ca2+](i) and isometric force si multaneously in an intact artery after a single addition of U46619. The ini tial transient phase was associated with Ca2+ release from the sarcoplasmic reticulum, whereas the maintained phase was associated with Ca2+ influx. T wo distinct types of Ca2+ sensitization characterized these phases with eit her protein kinase C (PKC)-mediated or Rho-kinase-mediated mechanisms. Thei r effects were quite distinct on the basis of the time courses over which t he sensitization was effective. PKC inhibition (1 mu mol/L calphostin C) ha d a much greater effect in the initial phase, diminishing the size of the t ransient and prolonging the rise in force and the decline in [Ca2+](i). The re were limited effects on the sustained force. Rho-kinase inhibition (10 m u mol/L Y27632), in contrast, nearly abolished the sustained force but had a lesser effect on the transient phase. Neither inhibitor had any effect on the force versus [Ca2+](i) relations for KCl contractures. Our evidence su ggests that both PKC-mediated and Rho-kinase-mediated Ca2+ sensitizations a re present in coronary arteries, but the latter is dominant in thromboxane A, receptor-mediated contraction.