Force velocity characteristics of oesophageal muscle: interaction of isoproterenol and calcium.

The purpose of this study was to evaluate the interaction of isoproterenol and calcium upon the force velocity characteristics of opposum oesophageal circular muscle, in vitro. Isotonic and isometric recordings were used to determine the velocity of shortening and force, respectively. All muscle was studied at its length of optimal tension development (Lo). To determine the effect of isoproterenol upon muscle contractility, studies were performed at 2.5 mM calcium Kreb's solution. The maximal velocity of shortening (V max) and the peak force (Po) of oesophageal muscle were 6.2 plus or minus 0.3 mm/s and 16.8 plus or minus 1.0 gm, respectively, at 2.5 mM calcium. Isoproterenol (10--5 M) decreased the V max to 4.9 plus or minus 0.6 mm/s and the Po to 9.8 plus or minus 1.3 gm (P less than .01). Increased calcium (5.0 mM), alone, did not alter either V max or Po. However, at 5.0 mM calcium, 10--5 isoproterenol reduced the V max to 6.0 plus or minus 0.6 mm/s (P less than .05) and the Po to 13.9 plus or minus 0.8 gm (P less than .05). These studies indicate that isoproterenol significantly reduces the V max and Po of oesophageal smooth muscle during the neural mediated "off response". This effect of isoproterenol is reversed, in part, by an increase in calcium concentration.