Heterometric regulation and calcium sensitivity of the infarcted rat heart.

1. The surviving hypertrophied muscle remaining after myocardial infarction in rats is less sensitive to extracellular Ca2+ than the normal myocardium. Since the inotropic effect of Ca2+ is modulated by sarcomere length, the present study was undertaken to determine if Ca2+ desensitization of infarcted left ventricles (LV) can be modulated by increasing the diastolic pressure (DP). 2. Rats submitted to left coronary artery ligation (N = 11) or sham-operation (N = 9) were killed 8-10 days later and their hearts perfused by the Langendorff technique. A balloon was introduced into the LV cavity to measure the isovolumic systolic pressure (ISP) produced by DP changes (0 to 25 mmHg) at three Ca2+ concentrations (0.8, 1.25 and 2.5 mM). 3. In control hearts submitted to a DP of 5 mmHg, the ISP increased from 36 +/- 3 to 63 +/- 4 and to 74 +/- 4 mmHg as external Ca2+ was changed from 0.8 to 1.25 and to 2.5 mM, respectively. In contrast, in infarcted hearts submitted to the same DP and Ca2+ concentrations, the ISP increased from 19 +/- 2 to 26 +/- 2 and to 27 +/- 3 mmHg. The depressed response to Ca2+ was not modified by increasing DP up to 25 mmHg, the greatest DP tested. At this DP, ISP increased from 75 +/- 4 to 103 +/- 5 and to 114 +/- 5 mmHg in control hearts and from 45 +/- 2 to 54 +/- 3 and to 55 +/- 4 mmHg in infarcted hearts.(ABSTRACT TRUNCATED AT 250 WORDS)