Luminal calcium regulates calcium release in triads isolated from frog and rabbit skeletal muscle

Abstract

Triads isolated from frog and rabbit skeletal muscle were equilibrated with different external [Ca2+], ranging from 0.025 to 10 mM. Vesicular calcium increased with external [Ca2+] as the sum of a linear plus a saturable component; the latter, which vanished after calsequestrin removal, displayed Bmax values of 182 and 132 nmol of calcium/mg of protein, with Kd values of 1.21 and 1.14 mM in frog and rabbit vesicles, respectively. The effect of luminal [Ca2+] on release kinetics in triads from frog and rabbit skeletal muscle was investigated, triggering release with 2 mM ATP, pCa 5, pH 6.8. In triads from frog, release rate constant (k) values increased sixfold after increasing luminal [Ca2+] from 0.025 to 3 mM. In triads from rabbit, k values increased 20-fold when luminal [Ca2+] increased from 0.05 to 0.7 mM. In both preparations, k values remained relatively constant (10-12 s-1) at higher luminal [Ca2+], with a small decrease at 10 mM. Initial release rates increased with luminal [Ca2+] in both preparations; in triads from rabbit the increase was hyperbolic, and in triads from frogs the increase was sigmoidal. These results indicate that, although triads from frog and rabbit respond differently, in both preparations luminal [Ca2+] has a distinctive effect on release, presumably by regulating sarcoplasmic reticulum calcium channels.

Relationship between depolarization‐induced force responses and Ca2+ content in skeletal muscle fibres of rat and toad.

“…Fig. 1), and such responses can be elicited many times provided that the fibre is repolarized in the high [K+] (Lamb & Stephenson, 1990a (Donoso, Prieto & Hidalgo, 1995;Sitsapesan & Williams, 1995). Measurement of total Ca2+ content of a muscle fibre After determining the response of a skinned muscle fibre to depolarization, the total amount of Ca2+ contained in the fibre could be quantified by pre-equilibrating the fibre in a solution with a particular [BAPTA] for 30 s and then transferring the fibre to a TX-oil emulsion in order to lyse all membranous compartments and release any Ca2+ within, as described by Fryer & Stephenson (1996).…”

Endoplasmic Reticulum Ca2+Handling in Excitable Cells in Health and Disease

“…Early studies in permeabilized hepatocytes provided evidence that an increase of intraluminal Ca 2ϩ levels increased the sensitivity of IP 3 receptors to IP 3 (Nunn and Taylor, 1992). Although subsequent studies confirmed a positive effect of intraluminal Ca 2ϩ on IP 3 receptors (Parys et al, 1993) (Donoso et al, 1995). Recordings of single RyR channels of native RyRs in SR vesicles in the presence of Mg-ATP using Cs ϩ as the charge carrier showed that raising luminal Ca 2ϩ concentration from 20 M to 5 mM increased the open channel probability .…”

Elementary and global aspects of calcium signalling.

“…1 C) (Lee, 1993). In addition to these cytosolic regulators of release, there are indications that the level of calcium within the lumen of the ER/SR might be capable of adjusting the sensitivity of both the InsP3Rs (Missiaen, Taylor & Berridge, 1992;Parys, Missiaen, De Smedt & Casteels, 1993;Horne & Meyer, 1995;Tanimura & Turner, 1996) and the RyRs (Donoso, Prieto & Hidalgo, 1995;Lukyanenko, Gyorke & Gyorke, 1996). Overloading cardiac myocytes results in a marked increase in the excitability of the RyRs, which is manifested by the appearance of regenerative calcium waves (Cheng, Lederer & Cannell, 1993;Cheng, Lederer, Lederer & Cannell, 1996a).…”