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Abstract
The effect of beta-adrenergic stimulation on tetanic tension (TT), maximal rate of rise of tension (+ TT) and phospholamban (PHL) phosphorylation were studied in the perfused rat heart. 3 ± 10−8 m isoproterenol perfused at different [Ca2+]o 0.25, 1.35 and 3.85 mm, significantly decreased TT while increased +TT and PHL phosphorylation at the three [Ca2+]o studied. Regression lines of the relationship between + TT and TT from individual data obtained at each [Ca2+]o in the presence and in the absence of isoproterenol, show that for the same level of +TT, TT is lower in the presence of isoproterenol, i.e. at high levels of PHL phosphorylation. The slopes of the lines were 0.137 s and 0.427 s (P< 0.05) in the presence and absence of isoproterenol respectively. The decrease in TT produced by the beta-agonist can be attributed to its relaxant action prevailing over its inotropic effect and may represent the mechanical expression of the enhanced phosphorylation of phospholamban.