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The relative contribution of sarcoplasmic reticulum (SR) and sarcolemma in the regulation of cardiac contractility (twitch force, the rate of contraction and the rate of relaxation) at different time periods (10, 20 and 30 minuted) and at elevated extracellular potassium (5.omM) was assessed  in the ventricular  tissues  of  catfish  and  toad using  adrenaline  as an  activator of SRCa2+ uptake, caffeine as an inhibitor of SRCa2+ uptake and verapamil as an inhibitor of sarcolemmal Ca2+  exchange. Cardiac contractility (twitch force, rate of contraction "df/dt" and the rate of relaxation – "df/dt") tended to decrease with increasing of the time periods from 10 to 20 and 30 minutes in the catfish and toad ventricular tissues at 2.5 and 5.omMK+, Elevated extracellular K+ had a positive inotropic effect on the twitch force developed at 20 and 30 minutes in the catfish cardiac muscle, but it had non significant negative effect in the toad cardiac muscle at the same periods. The df/dt developed after 30 minutes was significantly lower in the presence of 5.omMK+ than that developed at 2.5mMK+ in the catfish cardiac muscle, whereas the effect of elevated K+ on df/dt was similar to that of 2.5mMKO at all the time periods applied in the toad cardiac muscle. Elevated K+ had a negative inotropic effect on the-df/dt developed at all time periods applied in the catfish cardiac muscle, but it had no effect on the-df/dt developed in the toad cardiac muscle at the same time periods. Adrenaline, like caffeine had a positive inotropic effect on the twitch force, df/dt and – df/dt developed after all the time periods applied at 2.5 and 5.omMK+ in the ventricular tissues of both animals, relative to that of control. But, the positive inotropic effect of caffeine on the cardiac contractility was significantly lower than that of adrenaline at 2.5 and 5.omMK+ O and at the different time  periods applied (specially after 30 minutes) for  both  animals. Verapamil had a significantly negative inotropic effect on the cardiac contractility developed after all time periods applied at 2.5 and 5.omMK+ in both animals, relative to that of control. However the negative inotropic effect of verapamil on the cardiac contractility was significantly higher at elevated K+ than at 2.5mMK+. The twitch force developed after all time periods applied in the toad cardiac muscle was significantly lower than that in the catfish cardiac muscle after the same periods in either 2.5 or 5.omMK+. In the presence of adrenaline, the twitch force developed in the toad cardiac muscle after each period applied was non-significantly higher than that developed in the catfish cardiac muscle at 2.5 and 5.omMK,+ whereas it was significantly higher than that of catfish cardiac muscle in the presence of caffeine at 2.5 and 5.omMK+ after the same time periods. In the presence of verapamil, the twitch force developed after each time periods in the toad cardiac muscle was significantly and non- significantly higher than that of catfish cardiac muscle at 2.5 and 5.omMK+ respectively. The df/dt developed after each period applied in the catfish cardiac muscle was to somewhat similar to that developed in the toad cardiac muscle at 2.5 and 5.omMK+, but in the presence of adrenaline, it was significantly lower than that of toad cardiac muscle at 2.5 and 5.omMK+, whereas in the presence of caffeine, it was non-signifantly higher than that of toad cardiac muscle. In the presence of verapamil, the df/dt developed in the catfish cardiac muscle after the time period applied was significantly lower than that of toad cardiac muscle at 2.5 and 5.omMK+, also. The - df/dt developed in the catfish cardiac muscle after the time periods applied was similar to that of toad cardiac muscle at 2.5mMK+, but it was non-significantly lower than that of toad cardiac muscle at 5.omMK+. In the presence of adrenaline, the - df/dt developed after the time periods applied in the catfish cardiac muscle was significantly lower than that of the toad cardiac muscle at 2.5 and 5.omMK+, whereas the opposite situation was recorded in the presence of caffeine. In the presence of verapamil at 2.5mMK+, the-df/dt developed after the time periods applied in the catfish cardiac muscle was signifantly lower than that of the toad cardiac muscle, whereas it was similar in the cardiac muscle of both animals at 5.omMK+. So, it can be concluded that the cardiac contractility in the catfish and toad ventricular tissues is time dependent. Also, it seems that the sarcolemmal Ca2+ exchanges partake in the regulation of cardiac contractility in the ventricular tissue of both animal But, the contribution of sarcoplasmic reticulum in the regulation of cardiac force was higher in the toad cardiac muscle than that of catfish cardiac muscle. Elevated extracellular K+ had a positive inotropic effect on the cardiac contractility in both animals.

egyptian academic journal of biological sciences. c, physiology and molecular biology/egyptian academic journal of biological sciences. c, physiology and  molecular biology

The role of sarcoplasmic reticulum and sarcolemma in the regulation of cardiac contractility at different times and higher extracellur potassium in the cardiac muscle of catfish and toad.