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Methylxanthines and the contraction-relaxation cycle of isolated rat atria
49 METHYLXANTHINES AND THE CONTRACTION-RELAXATION CYCLE OF ISOLATED RAT ATRIA.K.Laustiola, T.Metsa-Ketela and J.Silvola. Dept.Biomed.Sci.Univ.Tampere, Tampere, Finland. Methylxanthines are known to inhibit CAMP phosphodiesterase and to exert a positive inotropic effect.It seems, however, as if the ability of a methylxanthine to produce one of the effects does not necessarily correlate with the ability to produce the other.Furthermore, an increase in the cardiac level of CAMP seems to relate better to relaxation than to positive inotropism (Metsa-Ketela, T., Acta Physiol.Scand., in press).We have studied the effects of 1-methyl-3-isobutylxanthine (MIX;O.OlmM), theophylline (THEO;lmM) and caffeine (CAF;lmM) on the contraction-relaxation cycle (CRC).Initially after 20 sec. CAF and THEO had the strongest inotropic effect.However, after 180 sec. MIX had the strongest inotropic effect, while CAF had the weakest.MIX and THEO had a marked chronotropic effect within 20 sec., while CAF was the weakest in this respect.MIX and THEO increased the parameters describing relaxation after 180 sec., whereas the effect of CAF was not significant.The CRC time was diminished by MIX and THEO, whereas CAF was ineffective.The PDE inhibition of these agents correlates well with their ability to intensify the relaxation phase and decrease the CRC time. Grant: The Orion and Medica Scientific Foundation, Finland.
ALAMETHICIN EFFECTS ON PHOSPHORYLATION OF A CARDIAC SARCOLEMMAL FRACTION PREDOMINANTLY CONSISTING OF RIGHT-SIDE-OUT VESICLES. .T:M.J. Lamers, Dept. of Biochemistry I, Erasmus University Rotterdam, The Netherlands. A highly purified SL preparation was isolated from canine heart by the method of Trumble et al. (Life Sciences 27, 207-214, 1980). The total Na+/K+-ATPase actiestimated by mild solubilization with SDS ranged from 150 to 200 pmol/hr/mg. The ATP-dependent Ca2+ uptake represented the activity of an inside-out subpopulation of the SL vesicles, as it could be differentiated from SR by the following criteria: no stimulation by oxalate, inhibition by external Na+ due to Na+/Ca2' antiporter action. At optimal concentration of the peptide ionophore alamethicin an 8-and 2fold activation of resp. Na+/K+-ATPase and Ca2+/Mg2+-ATPase was observed, suggesting that the preparation consists predominantly of right-side-out SL vesicles. An intrinsic CAMP-dependent protein kinase could be unmasked by alamethicin. Major 32P incorporation occurred into the 9000 D protein which previously (Biochim. Biophys. Acta 524, 443-459, 1980) was shown to be present both in SL and SR. Another substrate protein of M, 14 000 also became apparent but was undetectable in SR. The intrinsic Ca2+calmodulin-dependent kinase (substrate proteins 55 000 and 9000 D) was inactivated by alamethicin so that its latency could not be investigated. Protein phosphorylation appeared to be related to changes in the SL Ca2+/Mg2+-ATPase. (Supported by a grant from the Dutch Heart Foundation.)
ALAMETHICIN EFFECTS ON PHOSPHORYLATION OF A CARDIAC SARCOLEMMAL FRACTION PREDOMINANTLY CONSISTING OF RIGHT-SIDE-OUT VESICLES. .T:M.J. Lamers, Dept. of Biochemistry I, Erasmus University Rotterdam, The Netherlands. A highly purified SL preparation was isolated from canine heart by the method of Trumble et al. (Life Sciences 27, 207-214, 1980). The total Na+/K+-ATPase actiestimated by mild solubilization with SDS ranged from 150 to 200 pmol/hr/mg. The ATP-dependent Ca2+ uptake represented the activity of an inside-out subpopulation of the SL vesicles, as it could be differentiated from SR by the following criteria: no stimulation by oxalate, inhibition by external Na+ due to Na+/Ca2' antiporter action. At optimal concentration of the peptide ionophore alamethicin an 8-and 2fold activation of resp. Na+/K+-ATPase and Ca2+/Mg2+-ATPase was observed, suggesting that the preparation consists predominantly of right-side-out SL vesicles. An intrinsic CAMP-dependent protein kinase could be unmasked by alamethicin. Major 32P incorporation occurred into the 9000 D protein which previously (Biochim. Biophys. Acta 524, 443-459, 1980) was shown to be present both in SL and SR. Another substrate protein of M, 14 000 also became apparent but was undetectable in SR. The intrinsic Ca2+calmodulin-dependent kinase (substrate proteins 55 000 and 9000 D) was inactivated by alamethicin so that its latency could not be investigated. Protein phosphorylation appeared to be related to changes in the SL Ca2+/Mg2+-ATPase. (Supported by a grant from the Dutch Heart Foundation.)