A slow component of intramembranous charge movement during sarcoplasmic reticulum calcium release in frog cut muscle fibers.

Abstract

Cut muscle fibers from Rana temporaria were mounted in a double Vaseline-gap chamber and equilibrated with an end-pool solution that contained 20 mM EGTA and 1.76 mM Ca (sarcomere length, temperature,[14][15][16] Sarcoplasmic reticulum (SR) Ca release, A[CaT], was estimated from changes in myoplasmic pH (Pape, P.C., D.-S. Jong, and W.K. Chandler. 1995. J. Gen. Physiol. 106:259-336). The maximal value of A[CaT] obtained during a depleting depolarization was assumed to equal the SR Ca content before stimulation, [CasR]R (expressed as myoplasmic concentration). After a depolarization to -55 to -40 mV in fibers with [CasR] R ---= 1,000-3,000 ~M, currents from intramembranous charge movement, Icm, showed an early I~ component. This was followed by an I v hump, which decayed within 50 ms to a small current that was maintained for as long as 500 ms. This slow current was probably a component of Icm because the amount of OFF charge, measured after depolarizations of different durations, increased according to the amount of ON charge. Icm was also measured after the SR had been depleted of most of its Ca, either by a depleting conditioning depolarization or by Ca removal from the end pools followed by a series of depleting depolarizations. The early I~ component was essentially unchanged by Ca depletion, the I v hump was increased (for [CasR]a > 200 ~M), the slow component was eliminated, and the total amount of OFF charge was essentially unchanged. These results suggest that the slow component of ON/~m is not movement of a new species of charge but is probably movement of Qv that is slowed by SR Ca release or some associated event such as the accompanying increase in myoplasmic free [Ca] that is expected to occur near the Ca release sites. The peak value of the apparent rate constant associated with this current, 2-4%/ms at pulse potentials between -48 and -40 mV, is decreased by half when [Casa]a --= 500-1,000 ~xM, which gives a peak rate of SR Ca release of N5-10 ~tM/ms.

Kinetic isoforms of intramembrane charge in intact amphibian striated muscle.

“…Finally, approximation of a single two-state Boltzmann analysis to the charge-voltage curves suggested that RyR antagonists preserved the total charge Q~l,~x despite the kinetic changes. The findings thus parallel results from independent manipulations that altered q~ kinetics but preserved the distinct q~ and q~ transitions with their different properties, their total quantity, and their ON-OFF conservation, when sarcoplasmic reticular Ca 2+ content was depleted (to <10 IxM) or increased (long et al, 1995;Pape et al, 1996; but see also Polakova and Heiny, 1994). Such findings were compatible with an intramembrane qn charge primarily driven by tubular voltage change (Huang and Peachey, 1989;Huang, 1993Huang, , 1994aHuang, , 1994bHui and Chandler 1990).…”

“…This contrasts with the selective fall in ON charge reported by Garcia et al (1991) that suggested an action of ryanodine mediated solely through its effects on a release of intracellularly stored Ca 2+ that was in turn responsible for driving delayed charge movement. However, it parallels results from independent kinetic manipulations through sarcoplasmic reticular Ca 2+ depletion or loading, which conserved the intramembrane charge 0ong et Pape et al, 1996).…”

“…Yet, the present studies demonstrate that RyR modifications alter the kinetics particularly of q~ transients, even though the RyR resides in membranes of the terminal cisternae, from which the tubules are electrically isolated (see review in Huang, 1993). Recent depletions or loadings of sarcoplasmic reticular Ca 2+ have similarly accomplished kinetic changes while preserving the steady-state characteristics and individual identities of the component charge species (Jong et al, 1995;Pape et al, 1996). These suggest that altered conditions of intracellular Ca 2+ resulting from Ca 2+ release channel modification might cause such changes.…”

“…Additionally, RyR modification also preserved the maximum charge in, and the steepness (k -~ 7-9 mV) of, inactivation curves. Jong et al (1995) and Pape et al (1996) similarly reported persistently steep charge-voltage relationships (k ~ 7 mV) characteristic of a q~ contribution, whether fibers were depleted of, or loaded with, Ca z+.…”

FPL-64176 alters both charge movement and Ca 2+ release properties in amphibian muscle fibres

“…In particular, the q γ component of the intramembrane charge is thought to represent the electrical signature for conformational transitions in the DHPR-voltage sensor [21,28]. Its steady-state voltage dependence, pharmacological sensitivities particularly to DHPR antagonists [7,20], and its prolonged kinetics near threshold that sharply become more rapid with even slight further depolarization [1,31,38] closely parallel corresponding features of the release of intracellularly stored Ca 2+ [37]. A hypothesis in which such kinetic complexities reflect direct DHPR-RyR interactions would also explain their absence in systems such as cardiac and arthropod skeletal muscle in which a more indirect cellular triggering depends instead upon extracellular Ca 2+ entry [4,15,16,30].…”