Positive staircase effect in the rat heart.

The apparent lack of a positive staircase effect in ray myocardium may reflect inadequate metabolic support. Isolated rat hearts (n equals 10) were perfused at 37 degrees C with Krebs buffer containing 5 mM glucose. In 10 preparations increases in heart rate from 240 to 480/min resulted in twofold increases in left ventricular pressure and dP/dt. Pacing at a rate of 480/min resulted in mechanical deterioration of the preparation and in 50% decreases of myocardial ATP concentration within a 10-min period. Hearts of open-chest rats driven at the same rate for the same period maintained normal ATP stores. In isolated papillary muscles contracting isometrically at a rate of 30/min, peak stress 15g/mm2 (mean plus or minus SE, n equals 8) and was not changed by increasing the concentration of glucose from 5 to 30 mM. When frequency was raised from 30 to 300/min, stress declined to 5.0 plus or minus .15 g/mm2 in the presence of 5mM glucose (P smaller than .001) but increased to 8.8 plus or minus .21 g/mm2 (P smaller than 0.001) in the presence of 30 mM glucose. Thus, rat ventricular myocardium exhibits a positive staircase effect at physiological heart rates when metabolic support is adequate.