Pyruvate restores β-adrenergic sensitivity of L-type Ca²⁺ channels in failing rat heart: Role of protein phosphatase

Oxidative stress plays a major role in the pathogenesis of heart failure, where the contractile response to β-adrenergic stimulation is profoundly depressed. This condition involves L-type Ca²⁺ channels, but the mechanisms underlying their impaired adrenergic regulation are unclear. Thus the present study explored the basis for impaired adrenergic control of Ca²⁺ channels in a rat infarction model of heart failure. Patch-clamp recordings of L-type Ca²⁺ current (I_Ca,L) from ventricular myocytes isolated from infarcted hearts showed a blunted response to intracellular cAMP that was reversed by treatment with exogenous pyruvate. Biochemical studies showed that basal and cAMP-stimulated protein kinase A activities were similar in infarcted and sham-operated hearts, whereas molecular analysis also found that binding of protein kinase A to the α_1C subunit of voltage-gated Ca²⁺ channel isoform 1.2 was not different between groups. By contrast, protein phosphatase 2A (PP2A) activity and binding to α_1C were significantly less in infarcted hearts. The PP2A inhibitor okadaic acid markedly increased I_Ca,L in sham-operated myocytes, but this response was significantly less in myocytes from infarcted hearts. However, pyruvate normalized I_Ca,L stimulation by okadaic acid, and this effect was blocked by inhibitors of thioredoxin reductase, implicating a functional role for the redox-active thioredoxin system. Our data suggest that blunted β-adrenergic stimulation of ICaL in failing hearts results from hyperphosphorylation of Ca²⁺ channels secondary to oxidation-induced impairment of PP2A function. We propose that the redox state of Ca²⁺ channels or PP2A is controlled by the thioredoxin system which plays a key role in Ca²⁺ channel remodeling of the failing heart.