Background: An increase in cytosolic protein phosphatases (PPs) de-phosphorylates phospholamban, decreasing the Ca
2+ uptake of the sarcoplasmic reticulum (SR). The effects of PP inhibitors on cellular Ca
2+ handling were investigated.
Methods and Results: Twitch Ca
2+ transients (CaTs) and cell shortening were measured in intact rat cardiac myocytes, and caffeine-induced Ca
2+ transients (CaffCaTs) and Ca
2+ sparks were studied in saponin-permeabilized cells. Calyculin A augmented isoproterenol-induced increases in CaTs and cell shortening without altering the diastolic [Ca
2+]
i and twitch [Ca
2+]
i decay. The protein kinase A catalytic subunit (PKA
cat) increased the peak of CaffCaTs between 5 and 50 U/ml, and the addition of inhibitor-1 (I-1) augmented the increase. PKA
cat increased Ca
2+ spark frequency and the addition of I-1 increased it further. PKA
cat at 50 U/ml amplified the peak and prolonged the duration of Ca
2+ sparks, whereas the addition of I-1 did not alter them. An abrupt inhibition of SR Ca
2+ uptake following exposure to PKA
cat caused a gradual decrease in Ca
2+ spark frequency, but the addition of I-1 did not accelerate the decline of Ca
2+ spark frequency or CaffCaTs.
Conclusions: Inhibition of PPs augmented the inotropic effect of isoproterenol. Specific inhibition of PP1 could stimulate the Ca
2+ uptake of the SR with less significant effects on the Ca
2+ release. (
Circ J 2009;
73: 1133-1140)
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