Ca2+ load of guinea‐pig ventricular myocytes determines efficacy of brief Ca2+ currents as trigger for Ca2+ release.

Abstract

1. In guinea-pig ventricular cells, the concentration of ionized cytosolic calcium ([Ca2+]

Intrasarcomere [Ca2+] gradients in ventricular myocytes revealed by high speed digital imaging microscopy.

“…Myocytes were isolated from the ventricles of 400-g guinea pigs as described previously (13). Fluorescent Ca2+ indicators were loaded into single voltage-clamped guinea pig cardiac myocytes through a patch pipette.…”

All-or-None Ca2+Release from Intracellular Stores Triggered by Ca2+Influx through Voltage-Gated Ca2+Channels in Rat Sensory Neurons

“…The threshold decreased in parallel with the time course for refilling the stores, and inhibition of the Ca 2ϩ ATPases that sequester Ca 2ϩ into the stores increased the duration of the refractory period. Loading the store may determine the strength of the coupling between release sites by increasing the driving force for Ca 2ϩ through the open channel (Han et al, 1994). This explanation posits that the sensitivity of the ryanodine receptor to Ca 2ϩ does not change and that the decreased coupling can be compensated by elevated [Ca 2ϩ ] i .…”

“…This so-called "Ca 2ϩ synapse" (Stern, 1992) provides a mechanism of local control in which an increase in C a 2ϩ influx increases the number of independent release events that sum to produce graded CICR (Wier et al, 1994). Regenerative responses result when release sites are sufficiently coupled to enable the C a 2ϩ elevation to spread from one release unit to another, overcoming fast buffering processes (Lipp et al, 1992;Han et al, 1994). Thus, factors such as the density of ryanodine receptors, the level of C a 2ϩ within the stores, and the sensitivity of ryanodine receptors to cytosolic Ca 2ϩ determine whether stimulus-evoked sparks will sum to a regenerative response.…”

Mechanisms Underlying the Effects of the Pyrethroid Tefluthrin on Action Potential Duration in Isolated Rat Ventricular Myocytes

“…Interestingly, the first Ca 2ϩ wave during highly prolonged action potentials was delayed by over 100 ms with respect to stimulation, coinciding with early repolarization (to about ϩ10 mV (Sham, 2000). Therefore, the recovery of refractory Ca 2ϩ release channels during SR refilling may have permitted the threshold Ca 2ϩ load for spontaneous Ca 2ϩ release to be attained 100 to 200 ms after a stimulation (Han et al, 1994;Spencer and Berlin, 1995;Sham et al, 1998). Pseudoratio calibrations suggested a maximal [Ca 2ϩ ] i of about 1.6 M at the peak of Ca 2ϩ waves; and assuming that peak subsarcolemmal [Ca 2ϩ ] i was similar, such a concentration could represent a lower limit for subsarcolemmal [Ca 2ϩ ] i at the time of tefluthrin-modified I Na (Weber et al, 2002).…”