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Ouabain effects on calcium loading properties of sarcoplasmic reticulum in rat ventricular myocardium
j Mol Cell Cardiol 18 (Supplement 1) (1986) 171 FURTHER CHARACTERIZATION OF 5-HTI-LIKE RECEPTORS IN THE CAT HEART. Pramod R. Saxena and Ewan J. Mylecharane. Deportments of Phormocology, Erosmus University, Rotterdom, The Netherlonde nnd University of Sydney, Sydney, Auetrnlin. We recently reported that tachycardio elicited by 5-hydroxytryptamine (5-HT) in the spinal cat was antagonized by methyssrgide, but not by ketanserin, showing that 5-HT1-11ke receptors mediate this response. Further characterization of these receptors was attempted by comparing the effects of several antagonists and agonists. Besides methysergide, tachycardlc response to 5-HT was suppressed by methiothepin, metergoline, mesulergine, pizotifen and mianserin. The order of antagonist potency was: methiothepin >> methysergide > metsrgoline > mesulergine ~ pizotifen mianserin. Amongst the agonist drugs, 5-carboxamidotryptamine (5-CT) and N-(5-acetylaminophenyl) piperazine (BEA 1654) were quite effective in eliciting a methiothepin-sensitive but ketanserin-insensitive tachycardia in the spinal cat. 8-hydroxy-2-(di-N-propylamino)-tetralin (8-OH-DPAT) caused some tachycardia but 5-methoxy-5(1,2,5,6-tetrahydropyridin-4-yl)IH indele (RU 24969) was essentially ineffective. The order of agonist potency was: 5-CT > 5-HT >> BEA 1655 > 8-OH-DPAT >> RU-24969. However, agonist potencies in elioiting functional effects did not correlate with the affinities for either the 5-HTIA or 5-HTIB binding subsites, The involvement of the 5-HTIc subsite also appears unlikely because of poor antagonism by mesulergine. It is concluded that the cat heart 5-HT I receptor is apparently distinct from the subdivisions of 5-HT I binding sites demonstrated so far.
172REGIONAL AND SPECIES DIFFERENCES IN T H E ACTIVITY OF MYOCARDIAL 5'-NUCLEOTIDASE. Choong, Y.S. Department of Pathology, University of Auckland School of Medicine, Auckland, N e w Zealand. The present study was carried out to determine if the known struetural and metabolic differences between atrial and ventricular myoeardium could be explained by regional differences in the 5'-nueleotidase (5'NT) activity (a regulatory e n z y m e in the catabolic pathway of high energy phosphates to adenosine). Various standard samples from the atria and ventricles of dog and rat hearts were rapidly frozen in liquid nitrogen and processed for assay of 5'NT. In normal dog heart, the atrial tissue had 3 to 4 times the 5 ' N T activity of ventriealar tissue. Also the right atrial (11.35 + 1.48 nmol/min/mg protein) and right ventrieular (3.24 + 0.57 n m o l / m i n / m g protein) sit"as showed a two fold greater activity of 5 ' N T than the co~rasponding left sites (p ( 0 . 0 2 ) . In the rat heart, the ratio of the measured 5 ' N T activity in atrial to ventriealar myoeardium was 2. However, the total e n z y m e antivity was about 10 times higher in the rat than in the canine myoeardium. This data shows that in both species the atria have a greater eapaeity to breakdown A M P to adenosine than the ventricles. (Supported by the National Heart Foundation of N e w Zealand)
1 7 3 O U A B A I N EFFECTS ON CALCIUM LOADING PROPERTIES OF SARCOPLASMIC RETICULUM IN RAT VENTRICULAR MYOCARDIUM. L.M. Gibson & D.G. Stephenson, Department of Zoology, La Trobe University, Bundoora, Victoria. Functionally skinned cardiac preparations from the right ventricle of the rat were obtained by homogenization and loaded with Ca at room temperature as earlier described (Gibson& Stephenson, 1985). The SR was unloaded in a relaxing solution (0.04mY EGTA) with 30mM caffeine, and the magnitude of the force transient used as an indication of the amount of Ca stored by the SR. Previous results showed that rat cardiac SR exhibits a cyclic uptake and release of calcium at constant [Ca=+](Gibson & Stephenson, 1985). Due to this cyclic activity it was difficult to interpret the effects of various drugs such as dantrolene sodium on SR loading with Ca. In the presence of ouabain the magnitude of the force transients markedly increased compared to controls, indicating that more Ca was loaded by the SR. Ouabain also had a 'damping' effect on the cyclic Ca loading of the SR. The decreased cyclical activity of the SR in the presence of ouabain was further used to clarify results obtained with dantrolene sodium. Under these conditions Ca loading in the presence of dantrolene was substantially enhanced for all loading periods up to 5 mins. These effects can be explained by ouabain's influence on the polarization state of the membrane fragments in this preparation. Alternatively, ouabainmay exert its action by directly affecting the SR membranes. Gibson, L.M. & Stephenson, D.G. 1985. Proc. Aust. Physiol. Pharmacol. Soc., 16, 123p. (Supported by the National Heart Foundation of Australia)
172REGIONAL AND SPECIES DIFFERENCES IN T H E ACTIVITY OF MYOCARDIAL 5'-NUCLEOTIDASE. Choong, Y.S. Department of Pathology, University of Auckland School of Medicine, Auckland, N e w Zealand. The present study was carried out to determine if the known struetural and metabolic differences between atrial and ventricular myoeardium could be explained by regional differences in the 5'-nueleotidase (5'NT) activity (a regulatory e n z y m e in the catabolic pathway of high energy phosphates to adenosine). Various standard samples from the atria and ventricles of dog and rat hearts were rapidly frozen in liquid nitrogen and processed for assay of 5'NT. In normal dog heart, the atrial tissue had 3 to 4 times the 5 ' N T activity of ventriealar tissue. Also the right atrial (11.35 + 1.48 nmol/min/mg protein) and right ventrieular (3.24 + 0.57 n m o l / m i n / m g protein) sit"as showed a two fold greater activity of 5 ' N T than the co~rasponding left sites (p ( 0 . 0 2 ) . In the rat heart, the ratio of the measured 5 ' N T activity in atrial to ventriealar myoeardium was 2. However, the total e n z y m e antivity was about 10 times higher in the rat than in the canine myoeardium. This data shows that in both species the atria have a greater eapaeity to breakdown A M P to adenosine than the ventricles. (Supported by the National Heart Foundation of N e w Zealand)
1 7 3 O U A B A I N EFFECTS ON CALCIUM LOADING PROPERTIES OF SARCOPLASMIC RETICULUM IN RAT VENTRICULAR MYOCARDIUM. L.M. Gibson & D.G. Stephenson, Department of Zoology, La Trobe University, Bundoora, Victoria. Functionally skinned cardiac preparations from the right ventricle of the rat were obtained by homogenization and loaded with Ca at room temperature as earlier described (Gibson& Stephenson, 1985). The SR was unloaded in a relaxing solution (0.04mY EGTA) with 30mM caffeine, and the magnitude of the force transient used as an indication of the amount of Ca stored by the SR. Previous results showed that rat cardiac SR exhibits a cyclic uptake and release of calcium at constant [Ca=+](Gibson & Stephenson, 1985). Due to this cyclic activity it was difficult to interpret the effects of various drugs such as dantrolene sodium on SR loading with Ca. In the presence of ouabain the magnitude of the force transients markedly increased compared to controls, indicating that more Ca was loaded by the SR. Ouabain also had a 'damping' effect on the cyclic Ca loading of the SR. The decreased cyclical activity of the SR in the presence of ouabain was further used to clarify results obtained with dantrolene sodium. Under these conditions Ca loading in the presence of dantrolene was substantially enhanced for all loading periods up to 5 mins. These effects can be explained by ouabain's influence on the polarization state of the membrane fragments in this preparation. Alternatively, ouabainmay exert its action by directly affecting the SR membranes. Gibson, L.M. & Stephenson, D.G. 1985. Proc. Aust. Physiol. Pharmacol. Soc., 16, 123p. (Supported by the National Heart Foundation of Australia)