RESUMO
The activation of the Ca2+-independent (basal) ATPase from rat skeletal muscle microsomes is demonstrated in the presence of enough Ca2+ to provide the simultaneous activation of the (Ca2+ + Mg2+)-ATPase. It was achieved taking advantage of the delayed inorganic phosphate (Pi) release due to the formation of a phosphoenzyme complex during the Ca2+-dependent enzymatic cycle, which is evidenced in fast experiments. The microsomes were immobilized on a filter and perfused at constant flow with an incubation medium which was briefly interrupted with a pulse of appropriate reactants to activate the ATPases, at 2 degrees C. Successive samples were collected after passing through the filter, at approx. 0.1 s intervals. The Pi effluent profile coincides with the pattern of the pulse when it activates only the Ca2+-independent ATPase, it appears delayed when the pulse activates only extra Pi production by the (Ca2+ + Mg2+)-ATPase, and it includes a rapid and a delayed component when both Ca2+-independent and Ca2+-dependent ATPases are activated simultaneously by the pulse.
Assuntos
Adenosina Trifosfatases/metabolismo , ATPase de Ca(2+) e Mg(2+)/metabolismo , ATPases Transportadoras de Cálcio/metabolismo , Cálcio/farmacologia , Músculos/enzimologia , Trifosfato de Adenosina/metabolismo , Animais , Ativação Enzimática/efeitos dos fármacos , Microssomos/enzimologia , Fosfatos/metabolismo , RatosRESUMO
The effects of verapamil (0.01-0.1 mM) and Ca-free solutions (0 Ca, 3 mM MgCl2, 5 mM EGTA) upon mechanical and electrical properties of a fast twitch mammalian skeletal muscle (extensor digitorum longus) have been studied. Ca-free saline reduces twitch and tetanus tension. The response to single pulses was not affected in the presence of verapamil, but as the rate of stimulation increased, peak tensions were lower and a gradual decrease of tension was observed: frequency-dependent effect. Caffeine (30 mM) contracture was significantly reduced by both solutions. The fibers in Ca-free saline were depolarized (6 mV) and +dV/dt, -dV/dt and overshoot of the AP's were reduced. On the other hand, verapamil did not affect Vm. The presence of 0.1 mM verapamil hindered the ability of the fibers to generate AP's at high rates of stimulation. Lower concentration of verapamil (0.01) did not affect the repetitive electrical activity, but tetanic tension was decreased. These findings support partially the hypothesis about the dependence of mammalian skeletal muscle from external Ca. Verapamil would have several mechanisms of action: a) Ca-channel blocker action, b) local anaesthetic effect, and c) reduction of Ca release from the sarcoplasmic reticulum.
Assuntos
Contração Muscular/efeitos dos fármacos , Músculos/fisiologia , Retículo Sarcoplasmático/fisiologia , Verapamil/farmacologia , Potenciais de Ação/efeitos dos fármacos , Animais , Cafeína/farmacologia , Cálcio/fisiologia , Estimulação Elétrica , Músculos/efeitos dos fármacos , Ratos , Ratos Endogâmicos , Retículo Sarcoplasmático/efeitos dos fármacos , SoluçõesRESUMO
The effects of verapamil (0.01-0.1 mM) and Ca-free solutions (0 Ca, 3 mM MgCl2, 5 mM EGTA) upon mechanical and electrical properties of a fast twitch mammalian skeletal muscle (extensor digitorum longus) have been studied. Ca-free saline reduces twitch and tetanus tension. The response to single pulses was not affected in the presence of verapamil, but as the rate of stimulation increased, peak tensions were lower and a gradual decrease of tension was observed: frequency-dependent effect. Caffeine (30 mM) contracture was significantly reduced by both solutions. The fibers in Ca-free saline were depolarized (6 mV) and +dV/dt, -dV/dt and overshoot of the APs were reduced. On the other hand, verapamil did not affect Vm. The presence of 0.1 mM verapamil hindered the ability of the fibers to generate APs at high rates of stimulation. Lower concentration of verapamil (0.01) did not affect the repetitive electrical activity, but tetanic tension was decreased. These findings support partially the hypothesis about the dependence of mammalian skeletal muscle from external Ca. Verapamil would have several mechanisms of action: a) Ca-channel blocker action, b) local anaesthetic effect, and c) reduction of Ca release from the sarcoplasmic reticulum.