Bimodal effect of stimulation on light fluctuation transients monitoring spontaneous sarcoplasmic reticulum calcium release in rat cardiac muscle

Microscopic, myofilament motion caused by spontaneous oscillatory Ca²⁺ release from the sarcoplasmic reticulum (SR) of unstimulated rat papillary muscles produces scattered light intensity fluctuations (SLIF) in a laser beam scattered by the tissue. SLIF frequency increases with Ca²⁺ loading of resting muscle. We used novel time-gated SLIF measurements to determine how electrical stimulation (which per se both induces SR Ca²⁺ release and modulates total cellular Ca²⁺ loading) affects SLIF. Stimulation of thin rat, right ventricular muscles at 1 Hz in bathing [Ca²⁺] (Ca₀) of 1.5 mM at 29°C abolished SLIF for 5-7 seconds; SLIF then reappeared and monotonically increased for 10-15 seconds to reach the steady resting level. Resting force transients paralleled those of SLIF. The magnitude of depression and time course of recovery of both resting force and SLIF at this Ca₀ vary inversely with the rate of prior stimulation and the number of stimuli given. An increase in Ca₀ or disablement of the Na-K pump increased both resting SLIF and force; transient stimulation under these conditions (i.e., in a 2.5-5.5-second 'diastolic window' after cessation of stimulation) augmented SLIF and force above the resting level. Isoproterenol caused a modest reduction of resting SLIF, but it transiently increased SLIF after stimulation up to 10-fold above the resting level. Nifedipine did not affect resting SLIF but transiently depressed SLIF after stimulation. Ryanodine abolished SLIF both after stimulation and at rest. These results permit the inference that SLIF transients noninvasively monitor transient changes in the average extent of SR Ca²⁺ loading without necessitating test depolarizations and suggest that, depending on conditions, stimulation of rat muscle can either transiently increase or decrease the extent of SR Ca²⁺ loading.