Excitation-Contraction Coupling

Abstract

Since the classic experiments of Ringer (1883) demonstrated that frog heart would not contract in the absence of extracellular Ca, the crucial nature of Ca in muscle contraction has become increasingly clear. Figure 105A shows a modern version of Ringer’s experiment where Ca₀ is removed quickly from the medium around a rat ventricular myocyte causing an immediate abolition of contraction (in < 1 sec; Rich et al., 1988). This contrasts strikingly with skeletal muscle (Fig 105B) which can contract for many minutes in the complete absence of extracellular Ca (Armstrong et al., 1972). Figure 106A shows the E_m-dependence of several parameters during voltage-clamp of isolated guinea-pig ventricular myocytes. The E_m-dependence of Ca transients and contractions is bell-shaped, just like I_ca in cardiac preparations (McDonald et al., 1975; London & Krueger, 1986; Cannell et al., 1987; Beuckelmann & Wier, 1988; Callewaert et al., 1988; duBell & Houser, 1989). This is also true for an intrinsic birefringence signal in cardiac muscle which is thought to be associated with SR Ca release (Maylie & Morad, 1984). However, E_m-dependence of intramembrane charge movement (related to Ca channel activation) in heart is sigmoidal (Field et al., 1988; Bean & Ríos, 1989; Hadley & Lederer, 1989, 1991).