Role of protein phosphatases in regulation of cardiac inotropy and relaxation

We studied the effects of the protein phosphatase (PP) inhibitor cantharidi n (Cant) on time parameters and force of contraction (FOC) in isometrically contracting electrically driven guinea pig papillary muscles. We correlate d the mechanical parameters of contractility with phosphorylation of the in hibitory subunit of troponin (TnI-P) and with the site-specific phosphoryla tion of phospholamban (PLB) at serine-16 (PLB-Ser-16) and threonine-17 (PLB -Thr-17). Cant (after 30 min) started to increase FOC (112 +/- 4% of contro l, n = 10) and TnI-P and PLB-Thr-17 (120 +/- 5 and 128 +/- 7% of control) w ithout any alteration of relaxation time. Cant (10 muM) started to increase PLB-Ser-16, but the relaxation was shortened at only 100 muM (from 140 +/- 9 to 116 +/- 12 ms, n = 9). Moreover, 100 muM Cant, 3 min after applicatio n, started to increase PLB-Thr-17, TnI-P, and FOC. Cant (100 mM) began to i ncrease PLB-Ser-16 after 20 min. This was accompanied by shortening of rela xation time. Differences in protein kinase activation or different substrat e specificities of PP may explain the difference in Cant-induced site-speci fic phosphorylation of PLB in isometrically contracting papillary muscles. Moreover, PLB-Thr-17 may be important for inotropy, whereas PLB-Ser-16 coul d be a major determinant of relaxation time.