Mechanisms underlying afterload-induced exacerbation of myocardial infarct size: Role of T-Type Ca²⁺ channel

One consequence of elevated afterload pressure is the activation of the angiotensin II type 1 receptor and nonspecific cation channels with subsequent Ca²⁺ accumulation via the Na⁺/H⁺-Na ⁺/Ca²⁺ exchanger combination and the T-type or L-type Ca²⁺ channels. Intracellular Ca²⁺ overload is cytotoxic, in part, by inducing the mitochondrial permeability transition (MPT) pore. Therefore, we tested the hypotheses that: (1) increased afterload pressure worsens myocardial ischemia-reperfusion injury in healthy heart, (2) the Na ⁺/H⁺-Na⁺/Ca²⁺ exchanger combination and both the T-type and L-type Ca²⁺ channels are involved in the exacerbating impact of high afterload pressure on infarct size, and (3) elevated afterload enhances infarct size in part via the MPT pore. Accordingly, the effect of candesartan (angiotensin II type 1 receptor antagonist), cariporide (inhibitor of the Na⁺/H⁺ exchanger), mibefradil (T-type Ca²⁺ channel blocker), diltiazem (L-type Ca²⁺ channel blocker), or cyclosporine A (inhibitor of MPT pore) were examined. The elevation in afterload pressure from 80 to 160 cmH₂O increased baseline myocardial performance but caused larger infarcts and worsened recovery of mechanical function after ischemia reperfusion. Whereas mibefradil abrogated the effect of high afterload pressure on infarct size, the other agents reduced infarct size at both afterload pressures. Hearts exposed to mibefradil, diltiazem, or cariporide displayed greater functional recovery than those exposed to candesartan or cyclosporine A, revealing that an uncoupling exists between reduced cell death and recovery of mechanical function of the viable portions of the myocardium. The data also uncovered an important link between pressure-mediated exacerbation of infarct size and T-type Ca²⁺ channel activity.