Force‐calcium relationships in intact skeletal muscle: effects of temperature

Abstract

Force-Ca+2 relationships measured in isolated skeletal muscle cells are highly quantitative but can result in non-physiologic artifacts associated with isolation. We have developed a tissue fluorescence method to estimate ΔCa+2 while measuring tetanic force. Mouse hemidiaphragms were mounted in a cuvette that maintained fixed lateral and horizontal displacements and minimal vertical displacements. Muscles were loaded with 5 μM Rhod-2 AM, a cytosolic Ca+2 sensitive fluorophore. Excitation light was directed at 90° to emission using fiberoptic cables. As expected, low frequency stimulations (<60 Hz) resulted in greater peak forces at 23° C, whereas at 37° C, high frequency forces were elevated. In a given muscle, the relationship between ΔForce/ΔRhod-2 fluorescence at each stimulation frequency resolved to a single value at > 60 Hz, at both temperatures. In contrast, at < 60 Hz, the same peak Δfluorescence was observed with each contraction at both temperatures but at 37°C smaller changes in force were observed for a given Δ in Rhod-2 fluorescence. The data confirm that greater high frequency forces generated at 37°C are related to increased calcium release (NHLBI 53333).