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Antiarrhythmic effects of calcium antagonists
j Mol Cell Cardiol 18 (Supplement 3) (1986) MECHANISMS OF ACTION OF FORSKOLIN ON SKINNED AORTIC STRIPS.
J.Y.
Su,
C.Y.
Pan-Lee,
Department of Anesthesiology, RN-10, University of Washington School of Medicine, Seattle, WA 98195, USA. We hypothesized that forskolin induces relaxation of vascular smooth muscle by cAMP formation. We tested the hypothesis by comparing the effects of forskolin and cAMP on intraeellular sites of muscle contraction in skinned aortic strips. The medial layers of aortic strips isolated from rabbits were mounted an tension transducers and skinned with sapoain (500 u g/ml in relaxing solution) for 5 min. The preparations were immersed in bathing solutions in order to study the effects of the drugs on Ca 2+ activation of the contractile proteins, and Ca2~uptake and release from the sarcoplasmic reticulum (SR). We found tha~ 0.1 mM forskolin slightly increased, and cAMP slightly decreased the submaximal Ca 2+-, and both did not significantly change the maximal Ca2+-aetivated tension developma~t of the contractile proteins. At 1 ~M-0.1 mM, forskolin and cAMP decreased the Ca 2+ uptake by the SR by 10-46% and 0-84%, respectively, and the Ca 2+ release from the SR by 0-24% and 23-50%, respectively, in dose-dependent manner. Thus, the effects of forskolln on the SR are qualitatively similar to those of cAMP. The decreased Ca 2+ uptake by and release from the SR could be responsible for forskolin-lndueed relaxation of vascular smooth muscle. (Supported by grants in part from Washington Heart Association and in part from NIH #HL20754 and RCDA-HL01[00.)
ANTIARRHYTHMIC E F F E C T S OF C A L C I U M A N T A G O N I S T S . L. S z e k e r e s , J. Gy. Papp, ~ v a U d v a r y , A g n e s V~gh. D e p a r t m e n t of P h a r m a c o l o g y U n i v e r s i t y M e d i c a l S c h o o l of Szeged, H u n g a r y A f t e r an i n i t i a l o v e r e s t i m a t i o n of the a n t i a r r h y t h m i c efficiency a n d a p p l i c a b i l i t y of c a l c i u m a n t a g o n i s t s c l i n i c a l e x p e r i e n c e has s h o w n t h a t t h e i r use is e s s e n t i a l l y c o n f i n e d to t a c h y a r r h y t h m i a s of s u p r a v e n t r i c u l a r o r i g i n , an i n d i c a t i o n b a s e d on t h e i r d e p r e s s a n t a c t i o n on the sinoatrial and particularly the a t r i o v e n t r i c u l a r node. T h e r a p y of v e n t r i c u l a r a r r h y t h m i a s i n v o l v i n g s l o w c h a n n e l s is l i m i t e d b e c a u s e of the m a r k e d n e g a t i v e i n o t r o p i c a n d h y p o t e n s i v e e f f e c t of t h e s e d r u g s . O u r p r e s e n t e x p e r i m e n t s in w h i c h the "pure" C a 2 + - a n t a g o n i s t Verapamil compar e d w i t h the m i x e d /Ca 2+ and N a + - c h a n n e l b l o c k e r / a n t a g o n i s t s as f e n d i line or the K H L - s e r i e s of the C h i n o i n W o r k s on the r e l a t i o n s h i p b e t w e e n hypotensive and fibrillation threshold increasing and that between negative i n o t r o p i c a n d f i b r i l l a t i o n t h r e s h o l d i n c r e a s i n g E D 2 5 v a l u e s - h a v e s h o w n t h a t u s e of the m i x e d a n t a g o n i s t s m a y be m o r e f a v o u r a b l e . S i n c e a similar shift concerning relationship between hypotensive and antianginal-, viz. n e g a t i v e i n o t r o p i c a n d a n t i a n g i n a l E D 2 5 v a l u e s w a s o b s e r v e d , it is o b v i o u s t h a t m i x e d Ca 2+ a n t a g o n i s t s c o u l d be a d v a n t a g e o u s a g a i n s t a r r h y t h m i a s a s s o c i a t e d w i t h m y o c a r d i a l i s c h a e m i a , such as e a r l y a n d late ventricular postinfarction a r r h y t h m i a s as s h o w n in o u r e x p e r i m e n t s .
PROTEIN PHOSPHORYLATION IN CARDIAC SARCOPLASMIC RETICULUM AND ITS FUNCTIONAL CONSEQUENCES. M. TADA, M. KADOMA, J. FUJII. Division of Cardiology, First Department of Medicine, Osaka University School of Medicine, Osaka, Japan. Two classes of distinctly different phosphorylation were found to occur in proteins of cardiac sarcoplasmic reticulum (SR). One is an acyl phosphoprotein intermediate of Ca 2+ pump ATPase (EP), formed as a pivot step during translocation of Ca 2+ across SR. The other is a phosphoester formation in phospholamban (PN), catalyzed by cAMP- and calmodulin-dependent protein kinases (PK). Kinetic evidences indicate that PN phosphorylation accelerates Ca 2+ pump by enhancing the rates of E~P formation and decomposition, suggesting the existence of PN-mediated control over conformational changes of ATPase coupled with Ca translocation. Purified PN exhibits a unique molecular configuration that allows the reversible transition between 6-7K protomer (PNL) and 30K oligomer (PNH). While cAMP-PK-catalyzed phosphorylation occurs at serine in hydrophilic moiety of PN, the hydrophobic moiety should permit intermolecular contacts among PN species and with Ca 2+ pump ATPase within the membrane phospholipid milieu. Immunocytochemical findings obtained by light and electron microscopy, employing immunofluorescent and peroxidase-bound antibodies of PN, respectively, were consistent with the view that PN is a constituent element of cardiac SR, but not always of sarcolemma. It is intriguing to define whether Ca 2+mediated events in other contractile and non-contractile c e l l s are linked to the similar PN mechanisms.
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J.Y.
Su,
C.Y.
Pan-Lee,
Department of Anesthesiology, RN-10, University of Washington School of Medicine, Seattle, WA 98195, USA. We hypothesized that forskolin induces relaxation of vascular smooth muscle by cAMP formation. We tested the hypothesis by comparing the effects of forskolin and cAMP on intraeellular sites of muscle contraction in skinned aortic strips. The medial layers of aortic strips isolated from rabbits were mounted an tension transducers and skinned with sapoain (500 u g/ml in relaxing solution) for 5 min. The preparations were immersed in bathing solutions in order to study the effects of the drugs on Ca 2+ activation of the contractile proteins, and Ca2~uptake and release from the sarcoplasmic reticulum (SR). We found tha~ 0.1 mM forskolin slightly increased, and cAMP slightly decreased the submaximal Ca 2+-, and both did not significantly change the maximal Ca2+-aetivated tension developma~t of the contractile proteins. At 1 ~M-0.1 mM, forskolin and cAMP decreased the Ca 2+ uptake by the SR by 10-46% and 0-84%, respectively, and the Ca 2+ release from the SR by 0-24% and 23-50%, respectively, in dose-dependent manner. Thus, the effects of forskolln on the SR are qualitatively similar to those of cAMP. The decreased Ca 2+ uptake by and release from the SR could be responsible for forskolin-lndueed relaxation of vascular smooth muscle. (Supported by grants in part from Washington Heart Association and in part from NIH #HL20754 and RCDA-HL01[00.)
ANTIARRHYTHMIC E F F E C T S OF C A L C I U M A N T A G O N I S T S . L. S z e k e r e s , J. Gy. Papp, ~ v a U d v a r y , A g n e s V~gh. D e p a r t m e n t of P h a r m a c o l o g y U n i v e r s i t y M e d i c a l S c h o o l of Szeged, H u n g a r y A f t e r an i n i t i a l o v e r e s t i m a t i o n of the a n t i a r r h y t h m i c efficiency a n d a p p l i c a b i l i t y of c a l c i u m a n t a g o n i s t s c l i n i c a l e x p e r i e n c e has s h o w n t h a t t h e i r use is e s s e n t i a l l y c o n f i n e d to t a c h y a r r h y t h m i a s of s u p r a v e n t r i c u l a r o r i g i n , an i n d i c a t i o n b a s e d on t h e i r d e p r e s s a n t a c t i o n on the sinoatrial and particularly the a t r i o v e n t r i c u l a r node. T h e r a p y of v e n t r i c u l a r a r r h y t h m i a s i n v o l v i n g s l o w c h a n n e l s is l i m i t e d b e c a u s e of the m a r k e d n e g a t i v e i n o t r o p i c a n d h y p o t e n s i v e e f f e c t of t h e s e d r u g s . O u r p r e s e n t e x p e r i m e n t s in w h i c h the "pure" C a 2 + - a n t a g o n i s t Verapamil compar e d w i t h the m i x e d /Ca 2+ and N a + - c h a n n e l b l o c k e r / a n t a g o n i s t s as f e n d i line or the K H L - s e r i e s of the C h i n o i n W o r k s on the r e l a t i o n s h i p b e t w e e n hypotensive and fibrillation threshold increasing and that between negative i n o t r o p i c a n d f i b r i l l a t i o n t h r e s h o l d i n c r e a s i n g E D 2 5 v a l u e s - h a v e s h o w n t h a t u s e of the m i x e d a n t a g o n i s t s m a y be m o r e f a v o u r a b l e . S i n c e a similar shift concerning relationship between hypotensive and antianginal-, viz. n e g a t i v e i n o t r o p i c a n d a n t i a n g i n a l E D 2 5 v a l u e s w a s o b s e r v e d , it is o b v i o u s t h a t m i x e d Ca 2+ a n t a g o n i s t s c o u l d be a d v a n t a g e o u s a g a i n s t a r r h y t h m i a s a s s o c i a t e d w i t h m y o c a r d i a l i s c h a e m i a , such as e a r l y a n d late ventricular postinfarction a r r h y t h m i a s as s h o w n in o u r e x p e r i m e n t s .
PROTEIN PHOSPHORYLATION IN CARDIAC SARCOPLASMIC RETICULUM AND ITS FUNCTIONAL CONSEQUENCES. M. TADA, M. KADOMA, J. FUJII. Division of Cardiology, First Department of Medicine, Osaka University School of Medicine, Osaka, Japan. Two classes of distinctly different phosphorylation were found to occur in proteins of cardiac sarcoplasmic reticulum (SR). One is an acyl phosphoprotein intermediate of Ca 2+ pump ATPase (EP), formed as a pivot step during translocation of Ca 2+ across SR. The other is a phosphoester formation in phospholamban (PN), catalyzed by cAMP- and calmodulin-dependent protein kinases (PK). Kinetic evidences indicate that PN phosphorylation accelerates Ca 2+ pump by enhancing the rates of E~P formation and decomposition, suggesting the existence of PN-mediated control over conformational changes of ATPase coupled with Ca translocation. Purified PN exhibits a unique molecular configuration that allows the reversible transition between 6-7K protomer (PNL) and 30K oligomer (PNH). While cAMP-PK-catalyzed phosphorylation occurs at serine in hydrophilic moiety of PN, the hydrophobic moiety should permit intermolecular contacts among PN species and with Ca 2+ pump ATPase within the membrane phospholipid milieu. Immunocytochemical findings obtained by light and electron microscopy, employing immunofluorescent and peroxidase-bound antibodies of PN, respectively, were consistent with the view that PN is a constituent element of cardiac SR, but not always of sarcolemma. It is intriguing to define whether Ca 2+mediated events in other contractile and non-contractile c e l l s are linked to the similar PN mechanisms.
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