The influence of endothelin 1 on isometrically contracting human atrial muscle strip preparations was investigated under physiological conditions (37 degrees C, 1 Hz, Ca2+ 2.5 mM). Endothelin dose-dependently increased isometric tension from 3 x 10(-10) M to 1 x 10(-7) M. At 1 x 10(-7) M the inotropic effect of endothelin was maximum with isometric tension being increased by 32 +/- 6% (n = 11, p < 0.05). At 1 x 10(-7) M endothelin the positive inotropic effect was preceded by a transient negative inotropic effect with a decline in tension by -5 +/- 1% (n = 11, p < 0.05). Endothelin prolonged time from peak tension to 50% relaxation (RT50) by 29 +/- 5%. With BQ123 a competitive antagonist of the ETA receptor positive inotropic effect and the prolongation of relaxation was significantly reduced and initial negative a inotropic effect was abolished, indicating a ETA receptor mediated effect. Preincubation with phorbolmyristateacetate (10(-5) M) to downregulate proteinkinase C (PKC) eliminated the positive inotropic effect of endothelin. Similarly, N-5,5-dimethylamiloride (10(-5) M) which inhibits Na+/H(+)-exchanger activity, abolished the positive inotropic effect of ET. However, with either PMA or DMA the initial transient negative inotropic effect was still present (-13 +/- 7%, n = 9, p < 0.05 and -3 +/- 1%, n = 6, p < 0.05). Furthermore, both substances did not abolish the prolongation of twitch time parameters observed under endothelin. After preincubation with PMA, endothelin prolonged RT50 by 18 +/- 6% and with DMA by 11 +/- 2%. Using the photoprotein aequorin as an indicator for intracellular calcium concentrations showed that the positive inotropic effect was mainly mediated by an increase of systolic intracellular calcium concentrations. Thus, the present data indicate that the positive inotropic effect of endothelin in human atrial myocardium results from activation of PKC with a subsequent activation of the Na+/H(+)-exchanger. However, the initial negative inotropic effects as well as the prolongation of relaxation seem to result from a different intracellular mechanism of endothelin.