Protein Phosphatase Inhibitor-1 Augments a Protein Kinase A-Dependent Increase in the Ca2+ Loading of the Sarcoplasmic Reticulum Without Changing Its Ca2+ Release

Background: An increase in cytosolic protein phosphatases (PPs) de-phosphorylates phospholamban, decreasing the Ca2+ uptake of the sarcoplasmic reticulum (SR). The effects of PP inhibitors on cellular Ca2+ handling were investigated. Methods and Results: Twitch Ca2+ transients (CaTs) and cell shortening were measured in intact rat cardiac myocytes, and caffeine-induced Ca2+ transients (CaffCaTs) and Ca2+ sparks were studied in saponin-permeabilized cells. Calyculin A augmented isoproterenol-induced increases in CaTs and cell shortening without altering the diastolic [Ca2+]i and twitch [Ca2+]i decay. The protein kinase A catalytic subunit (PKAcat) increased the peak of CaffCaTs between 5 and 50 U/ml, and the addition of inhibitor-1 (I-1) augmented the increase. PKAcat increased Ca2+ spark frequency and the addition of I-1 increased it further. PKAcat at 50 U/ml amplified the peak and prolonged the duration of Ca2+ sparks, whereas the addition of I-1 did not alter them. An abrupt inhibition of SR Ca2+ uptake following exposure to PKAcat caused a gradual decrease in Ca2+ spark frequency, but the addition of I-1 did not accelerate the decline of Ca2+ spark frequency or CaffCaTs. Conclusions: Inhibition of PPs augmented the inotropic effect of isoproterenol. Specific inhibition of PP1 could stimulate the Ca2+ uptake of the SR with less significant effects on the Ca2+ release. (Circ J 2009; 73: 1133-1140)