SYNERGISTIC MODULATION OF ATP-SENSITIVE K-KINASE-C AND ADENOSINE - IMPLICATIONS FOR ISCHEMIC PRECONDITIONING( CURRENTS BY PROTEIN)

Ischemic preconditioning has been shown to involve the activation of a denosine receptors, protein kinase C (PKC), and ATP-sensitive K+ (K-AT P) channels. We investigated the effects of PKC activation and adenosi ne on K-ATP current (I-K,I-ATP) and action potentials in isolated rabb it ventricular myocytes. Responses to pinacidil (100 to 400 mu mol/L), an opener of K-ATP channels, were markedly increased by preexposure t o the PKC activator phorbol 12-myristate 13-acetate (PMA, 100 nmol/L). I-K,I-ATP measured at 0 mV was increased by PMA pretreatment from 0.5 5+/-0.32 to 3.25+/-0.47 nA (n=6, P<.01). We next determined whether PK C activation abbreviates the time required to turn on I(K,ATP) during metabolic inhibition (MI). In control cells in which MI was induced by 2 mmol/L cyanide and 0 glucose, I-K,I-ATP developed after an average of 15.1+/-2.4 minutes (n=8). Ten-minute pretreatment with PMA alone (P MA+MI) did not significantly alter this latency (11.9+/-2.0 minutes, n =8). Since adenosine receptor activation has been shown to play an imp ortant role in the preconditioning response, two groups of myocytes we re studied with adenosine (10 mu mol/L) included during MI. Without PM A, adenosine alone (MI+Ado) did not affect the latency to develop I-KA TP (12.3+/-1.5 minutes, n=8). However, if cells were pretreated with P MA and then subjected to MI in the presence of adenosine (PMA+MI+Ado), the latency was greatly shortened to 5.5+/-1.6 minutes (n=8; P<.02 ve rsus MI, PMA+MI, and MI+Ado groups). This effect could not be reproduc ed by an inactive phorbol but was completely abolished by the adenosin e receptor antagonist 8-(p-sulfophenyl)-theophylline. The opening of K -ATP, channels may be cardioprotective because of the abbreviation of action potential duration (APD) during ischemia. Therefore, we tested whether PKC activation could modify the time course of APD shortening during MI. Consistent with the ionic current measurements, PMA pretrea tment significantly accelerated APD shortening, but only when adenosin e (10 mu mol/L) was included during MI. The effects were not attributa ble to accelerated ATP consumption: PMA pretreatment did not alter the time required to induce rigor during MI, whether or not adenosine was included. Our results indicate that PKC activation increases the I-K, I-ATP induced by pinacidil or by MI. The latter effect requires concom itant adenosine receptor activation. The synergistic modulation of I-K ,I-ATP by PKC and adenosine provides an explicit basis for current par adigms of ischemic preconditioning.