Adenosine-sensitive alpha 1-adrenoceptor effects on reperfused ischaemic hearts: comparison with phorbol ester

1. We have examined the effects of the alpha 1-adrenoceptor agonists, phenylephrine or methoxamine, on contractility in rat and rabbit isolated hearts as well as their effects on postischaemic ventricular recovery. We compared these effects to those of 12-phorbol 13-myristate acetate (PMA), a direct activator of protein kinase C (PKC). 2. The positive inotropic effect of alpha 1-receptor agonists was significantly attenuated in the presence of the Na/H exchange inhibitor, methylisobutyl amiloride (MIA, 1 microM), whereas the positive inotropic effect of PMA was unaffected. 3. Reperfusion of rat hearts subjected to either 30 or 60 min of zero-flow ischaemia, resulted in recovery of contractility to 91 +/- 2% and 57 +/- 7% of the preischaemic values, respectively which was unaffected by phenylephrine. In contrast, PMA at a concentration (10 pM) devoid of direct depressant effects, significantly decreased recovery following 60 min of ischaemia to 31 +/- 4% of pre-ischaemic value (P < 0.05 from control); an effect which was completely prevented by the PKC inhibitor, bisindolylmaleimide. A similar inhibitory effect of PMA and lack of effect of phenylephrine were seen in reperfused rabbit hearts. 4. As alpha 1-receptor activation has been shown previously to stimulate cardiac adenosine production, we assessed whether blockade of adenosine A1 receptors with the specific antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 0.5 microM) would unmask the actions of phenylephrine in hearts subjected to 30 min ischaemia and reperfusion. In the presence of DPCPX, phenylephrine reduced recovery to 44 +/- 9% compared to 82 +/- 10% recovery in the absence of phenylephrine (P < 0.05). Identical results were observed in rabbit hearts treated with DPCPX in which recovery was reduced from 57.1 +/- 11.2% to 17.8 +/- 6.8% by phenylephrine (P < 0.05). Another A1 receptor antagonist, (+/-)-N6-endonorbornan-2-yl-9-methyladenine (N-0861, 0.5 microM) produced virtually identical results to those observed with DPCPX. 5. MIA failed to modulate the inhibition of postischaemic recovery by phenylephrine. Bisindolylmaleimide, on the other hand, partially prevented the effects of phenylephrine on postischaemic contractile dysfunction. The inhibitory effect of either PMA or phenylephrine on postischaemic recovery of both rat and rabbit hearts was generally dissociated from alterations in energy metabolism, although in the case of rat hearts, inhibition by phenylephrine was associated with diminished high energy phosphate content. 6. Our results demonstrate that both alpha 1-receptor activation as well as direct activation of PKC with phorbol ester can attenuate post-ischaemic ventricular recovery. Moreover, our results strongly suggest that endogenous adenosine protects the heart against the deleterious effects of alpha 1-receptor activation during ischaemia and reperfusion.