Distinct Pathways of Ca 2+ Sensitization in Porcine Coronary Artery

Abstract

Abstract —Alterations of the Ca 2+ sensitivity of contraction have been reported for porcine coronary artery, but the mechanisms are not clearly understood. We investigated the mechanism(s) of Ca 2+ sensitization in response to the thromboxane A 2 analogue (U46619). Our hypothesis is that different mechanisms of Ca 2+ sensitization could be distinguished by their distinct time courses. Therefore, we measured the time course of [Ca 2+ ] i and isometric force simultaneously in an intact artery after a single addition of U46619. The initial transient phase was associated with Ca 2+ release from the sarcoplasmic reticulum, whereas the maintained phase was associated with Ca 2+ influx. Two distinct types of Ca 2+ sensitization characterized these phases with either protein kinase C (PKC)-mediated or Rho-kinase–mediated mechanisms. Their effects were quite distinct on the basis of the time courses over which the sensitization was effective. PKC inhibition (1 μmol/L calphostin C) had a much greater effect in the initial phase, diminishing the size of the transient and prolonging the rise in force and the decline in [Ca 2+ ] i . There were limited effects on the sustained force. Rho-kinase inhibition (10 μmol/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 [Ca 2+ ] i relations for KCl contractures. Our evidence suggests that both PKC-mediated and Rho-kinase–mediated Ca 2+ sensitizations are present in coronary arteries, but the latter is dominant in thromboxane A 2 receptor–mediated contraction.