Distinct pathways of Ca(2+) sensitization in porcine coronary artery: effects of Rho-related kinase and protein kinase C inhibition on force and intracellular Ca(2+).

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 micromol/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 micromol/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.