Influences of stimulation frequency and temperature on interval-force relationships in guinea-pig papillary muscles

Relationships between contractile force and the preceding and pre-preceding stimulation intervals were studied in papillary muscles by interposing variable test intervals during steady-state pacing. The strength of test contractions increased exponentially to a maximum as the preceding (test) interval was lengthened. Contractility decreased as an exponential function of pre-preceding interval. At 37 degrees C, the half times for these processes were unaffected by increasing the steady-state frequency from 1 to 3 Hz. At 27 degrees C, the force increase with preceding interval was accelerated and the decay with pre-preceding interval was retarded as the stimulation frequency was increased from 0.33 to 2 Hz. The time-courses of force increase and decay were similar to each other during stimulation at an optimum frequency characteristic for the temperature. Cooling from 37 to 27 degrees C prolonged the half times for force increase and decay by factors of 4.5 and 3 respectively. The slope of the linear relationship between the force of the contraction pre-preceded by the test interval and the immediately subsequent contraction (recirculation fraction) was also halved. These results suggest that high stimulation frequency and low temperature uncouples cellular processes underlying the interval dependence of cardiac contractility. The temperature sensitivities are consistent with these processes being enzymatic. The reduced recirculation fraction provides a mechanism for the lowered threshold frequency for sustained mechanical alternans at 27 degrees C.