Blunted functional responses to pre- and postjunctional sympathetic stimulation in hibernating myocardium

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Abstract
Regional reductions in norepinephrine-tracer uptake are found in pigs with hibernating myocardium. Clinical studies would suggest that this is evidence for denervation; however, the functional responses to sympathetic stimulation have not been evaluated, and our previous studies with β-adrenergic stimulation have not suggested denervation hypersensitivity. Therefore, pigs were chronically instrumented to produce hibernating myocardium characterized by chronic regional dysfunction and histological viability. Open-chest studies were performed to determine changes in regional function in response to both pre- and postjunctional stimulation. Regional segment shortening was reduced at rest in hibernating myocardium compared with controls (13 ± 3% vs. 27 ± 3%, P = 0.004). During stellate ganglion stimulation, regional function increased in both groups of animals ( P = 0.008 vs. baseline), but the increase in hibernating myocardium was blunted compared with controls (Δ%, 3 ± 2% vs. 8 ± 3%, P = 0.04). Similar results occurred with intracoronary tyramine (10 μg/kg). Functional improvement during intravenous epinephrine infusion (0.35 μg·kg−1·min−1) was also blunted in hibernating myocardium compared with controls (Δ%, 7 ± 1% vs. 15 ± 2%, P = 0.04). Even when the improvement in function was expressed relative to the reduced baseline, there was no evidence for catecholamine-mediated hypersensitivity in hibernating myocardium. We therefore conclude that functional responses to both pre- and postjunctional sympathetic stimulation are blunted in pigs with hibernating myocardium. In contrast to previous studies of infarcted, denervated, and acutely stunned myocardium, there is no catecholamine-induced hypersensitivity in hibernating myocardium. These data suggest a downregulation in functional responses to stimulation that would protect hibernating myocardium from demand-induced ischemia at the expense of contractile reserve during sympathetic stimulation.