Depressed PKA activity contributes to impaired SERCA function and is linked to the pathogenesis of glucose-induced cardiomyopathy.

We have previously described a cardiomyopathy induced by culturing ventricular myocytes from normal adult rats in a medium containing high concentrations of glucose, which recapitulates cellular changes associated with early onset diabetic cardiomyopathy. This investigation was designed to evaluate cellular mechanisms that could contribute to slowed cytosolic Ca(2+) removal and myocyte relaxation in glucose-induced cardiomyopathy. Isolated ventricular myocytes were cultured overnight in medium containing normal glucose (n=5.5mM) or high glucose (HG=25.5mM). Cytosolic Ca(2+) removal was monitored with fluo-3 and myocyte mechanics with video-edge detection. Electrically stimulated Ca(2+) transients were prolonged in HG cells (A(T/PK)=215+/-7ms, n=41) compared to N myocytes (A(T/PK)=173+/-5ms, n=34). By pharmacological and ionic manipulations, Ca(2+) removal attributable to SERCA was slower in the HG group (A(D/PK)=290+/-17ms,n =41) compared to N (A(D/PK)=219+/-10, n=34), whereas NCX function was similar in both groups of cells. Total PKA activity was depressed in HG myocytes by 56% compared to N cells. beta-adrenergic receptor stimulation with ISO (10(-7)M) normalized myocyte relaxation, Ca(2+) transients and PKA activity in HG myocytes. Furthermore, inhibition of PKA with H89 (10(-5)M) depressed peak fractional shortening (PS) and slowed relengthening (A(R/PK)) to a greater extent in N (-50% for PS and 92% for A(R/PK)) than in HG cells (-25% for PS and 48% A(R/PK)). Depressed cytosolic Ca(2+) removal was not, however, associated with changes in basal levels of phosphorylated PLB, nor levels of SERCA, NCX or PLB proteins. We conclude that cellular mechanisms associated with the early onset glucose-induced cardiomyocyte dysfunction involves alterations in Ca(2+) regulation, which may be a common manifestation of other forms of cardiomyopathies.