INTRACELLULAR [CA2-FREQUENCY RELATIONSHIP OF LANGENDORFF PERFUSED RAT-HEART(] STAIRCASE IN THE ISOVOLUMIC PRESSURE)

Fluorescence and P-31 magnetic resonance spectroscopy have been used t o monitor simultaneously, the [Ca2+]i staircase and high energy phosph ate metabolism in isolated Langendorff-perfused rat heart paced at 2, 4 and 6 Hz. In order to investigate further the relationship between h igh energy phosphate metabolism and the calcium staircase we perturbed the intracellular phosphocreatine (PCr)/creatine concentration with d ietary beta-guanidinopropionic acid (beta-GPA). We have observed that: (a) At 2 Hz stimulation, the ventricular [Ca2+]i-dependent fluorescen ce decay is biexponential and continues to decay throughout the inters timulus interval; (b) at 4 Hz and 6 Hz, the [Ca2+] decay is monoexpone ntial; (c) end-diastolic [Ca2+]i is elevated at higher stimulation fre quencies; (d) net [Ca2+]i flux per cycle is reduced at higher stimulat ion frequencies and is therefore correlated inversely with stimulation frequency and end-diastolic [Ca2+]i; (e) ''heart rate [Ca2+]i flux product'' which is a measure of the work done in cycling calcium, is d irectly proportional to stimulation frequency; (f) the hysteresis betw een peal; ventricular isovolumic pressure and peak fluorescence is dec reased at higher stimulation frequencies: (g) no correlation was detec ted between the PCr/ATP ratio and stimulation Frequency; (h) despite a 60% decrease in the myocardial PCr/ATP ratio after beta-GPA feeding, rat heart is able to maintain the end-diastolic [Ca2+]i-dependent fluo rescence, and therefore the [Ca2+]i staircase relationship, similar to that of normal rat heart. In conclusion, using a physiological stimul ation range and substrate supply we have observed a negative staircase of both [Ca2+]i and isovolumic pressure in whole heart which is not h ypoxic. We propose that the inability of the sarcoplasmic reticulum to sequester sufficient cytosolic calcium at high stimulation frequencie s leads to an elevation in end-diastolic [Ca2+]i, decreased net calciu m flux per cycle resulting in a negative [Ca2+]i staircase and thus a negative isovolumic pressure-frequency relationship. We did not detect any correlation between steady-state high energy phosphate metabolism and stimulation frequency. (C) 1996 Academic Press Limited