Key result
Isoproterenol shifts the force-calcium curve rightward and decreases slope while EMD 53998 shifts it leftward.
Why the study?
The steady-state relationship between intracellular calcium concentration and force in intact cardiac muscle and its modulation by isoproterenol and EMD 53998 were not well characterized.
p-value: p=<0.02
Isoproterenol decreases cooperativity of thin filament activation, while EMD 53998 increases the affinity of strong-binding cross-bridges in intact rat cardiac muscle.
Suggests targeted myofilament sensitization without altered cooperativity; leaves open in vivo efficacy versus beta-agonists.
We have developed a novel method for measuring steady-state force-[Ca2+]i relations in isolated, membrane-intact rat trabeculae that are microinjected with Fura-2 salt. Twitches are markedly slowed after inhibition of phasic Ca2+ release and uptake from the sarcoplasmic reticulum by addition of cyclopiazonic acid and ryanodine. During relaxation of slowed twitches, force and [Ca2+]i trace a common trajectory in plots of force versus [Ca2+]i, despite very different histories of contraction. The common trajectory thereby provides a high resolution determination of the steady-state relation between force and [Ca2+]i. Using this method, we show that 1 microM isoproterenol, a beta-adrenergic agonist, causes a rightward shift (Hill function K1/2 increased from 0.39 +/- 0.07 microM to 0.82 +/- 0.23 microM, p < 0.02, n = 6) and a decreased slope (nH decreased from 5.4 +/- 1.1 to 4.0 +/- 1.4, p < 0.02) of the steady-state force-[Ca2+]i curve, with no change in maximal force (Fmax = 99.2 +/- 2.2% of control). In contrast, 2 microM EMD 53998, a racemic thiadiazinone derivative, causes a leftward shift (K1/2 decreased from 0.42 +/- 0.02 microM to 0.30 +/- 0.06 microM, p < 0.02, n = 4) with no change in slope of the steady-state force-[Ca2+]i curve, accompanied by a modest increase in maximal force (Fmax = 107.1 +/- 4.6% of control, p < 0.02). To gain mechanistic insight into these modulatory events, we developed a simple model of cooperative thin filament activation that predicts steady-state force-[Ca2+]i relationships. Model analysis suggests that isoproterenol decreases cooperativity arising from nearest-neighbor interactions between regulatory units on the thin filament, without change in the equilibrium constant for Ca2+ binding. In contrast, the effects of EMD 53998 are consistent with an increase in the affinity of strong-binding cross-bridges, without change in either the affinity of troponin C for Ca2+ or cooperative interactions.
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Dobrunz et al. (1995) studied Isolated rat trabeculae. Isoproterenol and EMD 53998 vs. Control was evaluated on Steady-state force-[Ca2+]i relationship (Hill function K1/2 and slope nH) (p=<0.02). Isoproterenol caused a rightward shift and decreased slope of the steady-state force-[Ca2+]i curve (p<0.02), whereas EMD 53998 caused a leftward shift with no change in slope (p<0.02).