Intracellular [Ca2+] staircase in the isovolumic pressure--frequency relationship of Langendorff-perfused rat heart

Fluorescence and 31P magnetic resonance spectroscopy have been used to monitor simultaneously, the [Ca2+]i staircase and high energy phosphate metabolism in isolated Langendorff-perfused rat heart paced at 2, 4 and 6 Hz. In order to investigate further the relationship between high energy phosphate metabolism and the calcium staircase we perturbed the intracellular phosphocreatine (PCr)/creatine concentration with dietary beta-guanidinopropionic acid (beta-GPA). We have observed that: (a) At 2 Hz stimulation, the ventricular -Ca2+-i-dependent fluorescence decay is biexponential and continues to decay throughout the interstimulus interval; (b) at 4 Hz and 6 Hz, the [Ca2+]i decay is monoexponential; (c) end-diastolic [Ca2+]i is elevated at higher stimulation frequencies; (d) net [Ca2+]i flux per cycle is reduced at higher stimulation 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 directly proportional to stimulation frequency; (f) the hysteresis between peak ventricular isovolumic pressure and peak fluorescence is decreased at higher stimulation frequencies; (g) no correlation was detected 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 fluorescence, and therefore the [Ca2+]i staircase relationship, similar to that of normal rat heart. In conclusion, using a physiological stimulation range and substrate supply we have observed a negative staircase of both [Ca2+]i and isovolumic pressure in whole heart which is not hypoxic. We propose that the inability of the sarcoplasmic reticulum to sequester sufficient cytosolic calcium at high stimulation frequencies leads to an elevation in end-diastolic [Ca2+]i, decreased net calcium 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.