Tension and Intracellular Calcium Transients of Activated Ferret Ventricular Muscle in Response to Step Length Changes

Abstract

To elucidate the effects of mechanical constraints on the (Ca²⁺) affinity of cardiac troponin C, we studied the relationships among the myoplasmic Ca²⁺ concentration ([Ca²⁺]_i), tension and length in steadily activated intact cardiac muscle. The Ca²⁺ sensitive photoprotein, aequorin, was micro-injected into cells of ferret right ventricular papillary muscles to monitor the [Ca²⁺]_i. The muscle was then steadily activated with ouabain (10^-4 M)(ouabain contracture) or high frequency stimuli in the presence of ryanodine (5 μM)(tetanic contraction); the tension and aequorin light (AL) transients in response to a step length change were then analyzed. The tension transient response to either the stretch or release in length was oscillatory: tension decreased rapidly during the release and then increased, after which it lapsed into a new steady level in a series of damped oscillations. The opposite was true for the stretch. The oscillatory responses were conspicuous and less damped in the ouabain contracture. The transient AL response was also oscillatory, the time course of which corresponded exactly to that of the tension transient response, though no detectable changes in AL were observed at the initial phase of the stretch response. The increase in AL corresponded exactly to the decrease in tension, likewise the decrease in AL to the increase in tension. The steady level of AL after release was decreased in ouabain contracture, but was increased in tetanic contraction. These results suggest that the Ca²⁺ affinity of cardiac troponin C is increased with an increase in tension (i.e., the cross-bridge attachment) and decreased with a decrease in tension (i.e., the cross-bridge detachment), and that the myoplasmic calcium concentration is lowered by release, at least in a Ca²⁺-overloaded condition, mainly through the sarcoplasmic reticulum.