Free Calcium in Rat Papillary Muscle at Contraction Assessed with Ca-Selective Microelectrodes

Abstract

Direct measurements of free intracellular calcium (Ca)i are needed for an understanding of the regulation of contractility. An on-line measurement of (Ca)i with Ca-selective microelectrodes in intact muscle strips provides a suitable means of investigating this problem, although considerable methodologic difficulties exist. Measurements of (Ca)i concentrations during muscle contraction have been carried out by different methods such as Ca-binding techniques and aequorin luminescence, but remain unsatisfying, since they were not performed on intact muscle strips. The authors' measurements were carried out with Ca-selective microelectrodes on rat papillary muscles (stretched to optimal length in a perfusion bath of 1.54 mL at 30°C). The impalement of electrodes was considered adequate when the heights of Ca-electrode potential and membrane potential remained constant for more than twenty minutes. For provoking contractile responses, the authors replaced the normal Tyrode solution by a caffeine-containing contracture solution (content in mM: 0 NaCl [choline], 4 CaCl₂, 30 KCI, 25 caffeine, 1.05 MgCl₂). Ca-se-

lective microelectrodes were calibrated before and after each measurement and only those impalements were taken as adequate that showed identical calibration curves before and after the experiment.

They measured the (Ca)i at 20%, 50%, and 80% of maximal contractile force and obtained (Ca)i concentrations of 1.1 ± 0.3 μM (at 20%), 3.6 ± 1.2 μM (at 50%), and 11.8 ± 0.27 μM (at 80%) (n=6, ±SEM).

These results represent the first on-line measurements of the myocardial (Ca)i concentrations with Ca⁺⁺selective microelectrodes in intact muscle strips during various degrees of contraction. It can be seen that contractile response does not occur below a (Ca)i concentration of 1 μM. Maximal contractile response is reached at (Ca)i concentrations beyond 10 μM.