Birefringence signals in mammalian and frog myocardium. E-C coupling implications.

Birefringence signals from mammalian and frog hearts were studied. The period between excitation and the onset of contraction in which optical signals were free of movement artifact was determined by changes in scattered incandescent light and changes in laser diffraction patterns. The birefringence signal preceding contraction was found to behave as a change in retardation and was not contaminated measurably by linear dichroic or isotropic absorption changes. There were two components of the birefringence signal in mammalian heart muscles but only one component in the frog heart. The first component of the birefringence signals in both mammalian and frog hearts had a time course coincident with the action potential upstroke. The second component in mammalian preparations was sensitive to inotropic interventions, such as variation of extracellular Ca2+, stimulation frequency, temperature, and epinephrine, in a manner that correlated with the maximum rate of rise of tension. Caffeine (2-10 mM) not only failed to generate a second component in the frog heart, but also suppressed the second component in the mammalian heart while potentiating twitch tension. The results suggest that the second component of the birefringence signal in the mammalian myocardium is related to Ca2+ release from the sarcoplasmic reticulum.