Heart rate modulates the slow enhancement of contraction due to sudden left ventricular dilation

In isovolumic blood-perfused dog hearts, left ventricular developed pressur e (DP) was recorded while a sudden ventricular dilation was promoted at thr ee heart rate (HR) levels: low (L: 52 +/- 1.7 beats/min), intermediate (M: 82 +/- 2.2 beats/min), and high (H: 117 +/- 3.5 beats/min). DP increased in stantaneously with chamber expansion (Delta 1DP), and another continuous in crease occurred for several minutes (Delta 2DP). HR elevation did not alter Delta 1DP (32.8 +/- 1.6, 33.6 +/- 1.5, and 34.3 +/- 1.2 mmHg for L, M, and H, respectively), even though it intensified Delta 2DP (17.3 +/- 0.9, 20.7 +/- 1.0, and 26.8 +/- 1.2 mmHg for L, M, and H, respectively), meaning tha t the treppe phenomenon enhances the length dependence of the contraction c omponent related to changes in intracellular Ca2+ concentration. Frequency increments reduced the half time of the slow response (82 +/- 3.6, 67 +/- 2 .6, and 53 +/- 2.0 s for L, M, and H, respectively), while the number of be ats included in half time increased (72 +/- 2.9, 95 +/- 2.9, and 111 +/- 3. 2 beats for L, M, and H, respectively). HR modulation of the slow response suggests that L-type Ca2+ channel currents and/or the Na+/Ca2+ exchanger pl ays a relevant role in the stretch-triggered Ca2+ gain when HR increases in the canine heart.