DUAL EFFECTS OF TETRACAINE ON SPONTANEOUS CALCIUM-RELEASE IN RAT VENTRICULAR MYOCYTES

1. Confocal microfluorometry was used to study the effects of tetracai ne on spontaneous Ca2+ release from the sarcoplasmic reticulum (SR) in isolated rat ventricular myocytes. 2. At low concentrations (0.25-1.2 5 mM), tetracaine caused an initial inhibition of spontaneous release events (Ca2+ sparks) and Ca2+ waves, which was followed by a gradual i ncrease in Ca2+ release activity. The frequency and magnitude of spark s were first decreased and then increased with respect to control leve ls. At high concentrations (> 1.25 mM), tetracaine abolished all forms of spontaneous release. 3. Exposure of the myocytes to tetracaine res ulted in a gradual increase in the SR Ca2+ load as indexed by changes in the magnitude of caffeine-induced Ca2+ transients. 4. In cardiac SR Ca2+-release channels incorporated into lipid bilayers, tetracaine (> 0.25 mM) induced a steady inhibition of channel activity. Addition of millimolar Ca2+ to the luminal side of the channel caused an increase in channel open probability under control conditions as well as in th e presence of various concentrations of tetracaine. 5. We conclude tha t the primary effect of tetracaine on SR Ca2+-release channels is inhi bition of channel activity both in vitro and in situ. The ability of t etracaine to reduce spark magnitude suggests that these events are not due to activation of single channels or nonreducible clusters of chan nels and, therefore, supports the multichannel origin of sparks. We pr opose that the paradoxical late potentiation of release by submaximal concentrations of tetracaine is caused by a gradual increase in SR Ca2 + load and subsequent activation of the Ca2+-release channels by Ca2inside the SR.