Inhibition of L-type Ca2+ channel current and negative inotropy induced by arachidonic acid in adult rat ventricular myocytes

We have previously shown an increase in arachidonic acid (AA) release in response to proinflammatory cytokines in adult rat ventricular myocytes (ARVM). AA is known to alter channel activities; however, its effects on cardiac L-type Ca(2+) channel current (I(Ca,L)) and excitation-contraction coupling remain unclear. The present study examined effects of AA on I(Ca,L), using the whole cell patch-clamp technique, and on cell shortening (CS) and the Ca(2+) transient of ARVM. I(Ca,L) was monitored in myocytes held at -70 mV and internally equilibrated and externally perfused with Na(+)- and K(+)-free solutions. Exposure to AA caused a voltage-dependent block of I(Ca,L) concentration dependently (IC(50) 8.5 microM). The AA-induced inhibition of I(Ca,L) is consistent with its hyperpolarizing shift in the voltage-dependent properties and reduction in maximum slope conductance. In the presence of AA, BSA completely blocked the AA-induced suppression of I(Ca,L) and CS. Intracellular load with AA had no effect on the current density but caused a small depolarizing shift in the I(Ca,L) activation curve, suggesting a site-specific action of AA. Moreover, intracellular AA had no effect on the extracellular AA-induced decrease in I(Ca,L). Pretreatment with indomethacin, an inhibitor of cyclooxygenase, or addition of nordihydroguaiaretic acid, an inhibitor of lipoxygenase, had no effect on AA-induced changes in I(Ca,L). Furthermore, AA suppressed CS and Ca(2+) transients of intact ARVM with no significant effect on SR function and myofilament Ca(2+) sensitivity. Therefore, these results suggest that AA inhibits contractile function of ARVM, primarily due to its direct inhibition of I(Ca,L) at an extracellular site.