Specific inhibition of protein phosphatase 1 by inhibitor-1 augmented the protein kinase A-dependent increase in sarcoplasmic reticulum Ca2+ loading without directly altering Ca2+ release.
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.
Kawashima et al. (Thu,) conducted a other in Heart failure (experimental model). Protein Phosphatase Inhibitor-1 (I-1) and Calyculin A vs. Control perfusion was evaluated on Sarcoplasmic reticulum Ca2+ loading and Ca2+ spark frequency. Specific inhibition of protein phosphatase 1 by inhibitor-1 augmented the protein kinase A-dependent increase in sarcoplasmic reticulum Ca2+ loading without directly altering Ca2+ release.