Halothane increases Ca2+ efflux via Ca2+ channels of sarcoplasmic reticulum in chemically skinned rat myocardium

1. A method has been developed to study Ca2+ fluxes across the sarcoplasmic reticulum (SR) of chemically (saponin) skinned myocardium without interference from the SR Ca2+ pump. 2. Exposure of rat cardiac trabeculae to a solution containing 50 micrograms/ml saponin for 10 min or longer caused an SR Ca2+ efflux which was not blocked by Ruthenium Red (RRed) and did not require the presence of nucleotides. 3. Exposure of the saponin-treated cardiac preparation to 11 mM-AMP, when the SR Ca2+ pump was not active, enhanced Ca2+ release from the SR by a mechanism which was blocked by 10 microM-RRed. 4. The amount of Ca2+ loaded by the 10 min saponin-treated trabeculae was maintained constant for at least 3 min when the preparations were transferred to low [Ca2+] solutions (0.1 mM-EGTA; pCa greater than 7.5) containing ATP. This indicated that the Ca2+ pump can efficiently recycle Ca2+ lost from the SR under these conditions. 5. Halothane (0.47 and 1.89 mM) reversibly increased the rate of Ca2+ release from the SR regardless of whether or not the SR Ca2+ pump was active. This effect was more marked at 1.89 mM than at 0.47 mM. RRed (10 microM) completely blocked the Ca2+ release induced by both concentrations of halothane. 6. The presence of nucleotide (11 mM-AMP) did not affect the halothane-induced Ca2+ release when the Ca2+ pump was inactive. 7. Exposure of cardiac preparations to solutions containing more than 5 mM-halothane irreversibly damaged the ability of the SR to load Ca2+. 8. The results suggest that at lower doses (0.47 and 1.89 mM) halothane specifically and reversibly stimulates Ca2+ efflux via the RRed-sensitive SR Ca2(+)-release channel by a mechanism which does not require the presence of nucleotides or relatively high [Ca2+]. The results also suggest that AMP and halothane act independently and non-synergistically to increase Ca2+ efflux through the same SR Ca2(+)-release channel. At higher doses (greater than 5 mM) halothane irreversibly damages the SR membrane, presumably by disrupting the lipid bilayer.