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 Ca(2+) uptake of the sarcoplasmic reticulum (SR). The effects of PP inhibitors on cellular Ca(2+) handling were investigated.


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.
Division of Cardiology, Internal Medicine III, Hamamatsu University School of Medicine, Hamamatsu 431-3192, Japan.
Hirotaka
H
Kawashima
Hiroshi
H
Satoh
Masao
M
Saotome
Tsuyoshi
T
Urushida
Hideki
H
Katoh
Hideharu
H
Hayashi
D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism / Q000494:pharmacology*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006706:Homeostasis / Q000187:drug effects / Q000502:physiology; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D008387:Marine Toxins; D023421:Models, Animal; D032383:Myocytes, Cardiac / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism*; D010080:Oxazoles / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D011506:Proteins / Q000378:metabolism / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology
D000318:Adrenergic beta-Agonists; D004791:Enzyme Inhibitors; D008387:Marine Toxins; D010080:Oxazoles; D011506:Proteins; C073882:phosphoprotein phosphatase inhibitor 1; C059041:calyculin A; D017868:Cyclic AMP-Dependent Protein Kinases; D007545:Isoproterenol; D002118:Calcium