LOW CONCENTRATIONS OF UD-CG-212 ENHANCE MYOCYTE CONTRACTILITY BY AN INCREASE IN CALCIUM RESPONSIVENESS IN THE PRESENCE OF INORGANIC-PHOSPHATE

Recent data show that UD-CG 212 in nanomolar concentrations increases myofibrillar Ca++ responsiveness of chemically skinned cardiac prepara tions in the presence of elevated inorganic phosphate. We studied the effects of UD-CG 212 on cell shortening of intact myocytes and in addi tion measured the intracellular calcium transients with the aid of IND O-1 fluorescence in the presence of 5 mM inorganic phosphate. The vali dity of our experimental system was first tested with the calcium chan nel opener Bay k 8644. Bay k 8644 at 10(-8) M did not significantly in fluence myocyte shortening(+ 13.9 +/- 4.6%, n = 9) but at 10(-7) M and 10(-6) M significantly increased contraction by 40.1 +/- 13.6% and 52 .5 +/- 17.0% respectively. Bay k 8644 at 10(-8) M increased the INDO-1 fluorescence ratio by 17.3) +/- 4.7% (P < 0.01; n = 9), and at 10(-7) M by 21.5 +/- 4.3% (P < 0.01; n = 9), whereas 10(-6) M Bay k 8644 had no significant effect on peak INDO-1 ratio. However, 10(-7) and 10(-6 ) M Bay k 8644 accelerated and broadened the calcium transients. Cell shortening of guinea pig ventricular myocytes electrically stimulated at 1 Hz was significantly increased by UD-CG 212 (10(-9)-10(-7) M) and isoprenaline (3 x 10(-8) (M)). An increase of 37.0 +/- 14.0% (P < 0.0 5; n = 9) was observed at 10(-9) M UD-CG 212, 90.5 +/- 18.2% (P < 0.05 ; n = 9) at 10(-8) M UD-CG 212, 164.0 +/- 34.9% (P < 0.05; n = 9) at 1 0(-7) M UD-CG 212, and 258.2 +/- 67.4% (P < 0.05; n = 9) at 3 x 10(-8) M isoprenaline. Peak INDO-1 fluorescence ratios were not significantl y (P > 0.05) influence after addition of 10(-9) M and 10(-8) M UD-CG 2 12, but significantly increased by 19.4 +/- 4.9% (P < 0.05; n = 9) at 10(-7) M UD-CG 212 and by 81.1) +/- 11.1% (P < 0.05; n = 9) at 3 x 10( -8) M isoprenaline. In conclusion, UD-CG 212 (10(-9)-10(-7) M) concent ration-dependently increased myocyte shortening in the presence of 5 m M inorganic phosphate. Low concentrations of 10(-9) and 10(-8) M UD-CG 212 increased myocyte contractility without altering the peak INDO-1 fluorescence ratio whereas 10(-7) M UD-CG 212 and 3 x 10(-8) M isopren aline increased cell shortening as well as peak INDO-1 fluorescence ra tio. These data suggest that low concentrations of UD-CG 212 increase myocyte contractility by enhancing myofibrillar calcium responsiveness whereas higher concentrations elevate intracellular calcium probably via increased intracellular cAMP brought about by phosphodiesterase in hibition.