RESUMEN
We have investigated the effect of the venom of a Chilean tarantula, Phrixotrichus spatulatus, on cell contraction, intracellular [Ca2+] ([Ca2+]i), and the L-type Ca2+ current (ICa,L) in cells isolated from the ventricles of guinea pig hearts. Whole-cell voltage clamp techniques were used to monitor ICa,L. The action potential was recorded using whole cell current clamp. [Ca2+]i was monitored using the fluorescent indicator indo-1. The venom of P. spatulatus decreased ICa,L in a dilution-dependent manner, with half-maximal inhibition at a dilution of 1.1/10(4) (v/v). At a dilution of 1/10(4), this inhibition occurred at all potentials so that the current-voltage relationship of ICa,L was depressed. However, inhibition of ICa,L by the venom was relieved by positive potentials, the venom being less effective following pulses to positive potentials. The venom reduced the duration of the action potential (at 50% repolarization) by between 26 and 43%. The venom also decreased the amplitude of the [Ca2+]i transient and cell contraction. It is concluded that the venom of P. spatulatus is a potent, voltage-dependent inhibitor of ICa,L; this inhibition of ICa,L may account for the effects of the venom on action potential duration, [Ca2+]i, and contraction.
Asunto(s)
Potenciales de Acción/efectos de los fármacos , Bloqueadores de los Canales de Calcio/farmacología , Calcio/metabolismo , Corazón/efectos de los fármacos , Venenos de Araña/farmacología , Animales , Células Cultivadas , Cobayas , Corazón/fisiología , Técnicas de Placa-ClampRESUMEN
Acidosis inhibits Ca2+ transport by the sarcoplasmic reticulum of cardiac muscle and decreases the Ca2+ sensitivity of the contractile proteins, although the mechanisms underlying these changes are unclear. We have investigated the hypothesis that changes in the phosphorylation of the regulatory proteins phospholamban and troponin I might play a role in the acidosis-induced changes in the function of the sarcoplasmic reticulum and the myofilaments, respectively. Langendorff-perfused rat hearts were labeled with 32P and then perfused with either control (pH 7.4) or acid (pH 6.8) physiological salt solution, in both the absence and presence of isoproterenol. The incorporation of 32P into phospholamban and troponin I was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of sarcoplasmic reticulum and myofibrillar proteins, followed by autoradiography and liquid scintillation counting. The data show that acidosis has no effect on the phosphorylation of phospholamban in the absence of isoproterenol but that, in the presence of isoproterenol, acidosis increased the phosphorylation of phospholamban. However, acidosis increased the phosphorylation of troponin I, in both the absence and the presence of isoproterenol. Acidosis did not alter the adenosine 3',5'-cyclic monophosphate content of the hearts but did inhibit type 1 phosphatase. These data show that acidosis can alter the phosphorylation of these two proteins and suggest that these changes underlie, in part the changes observed in cardiac muscle during acidosis.