Sarcoplasmic reticulum function abnormalities in rabbit failing hearts.

In a model of heart failure induced in rabbits by a double volume plus pressure overload, sarcoplasmic reticulum (SR) function was measured by Ca uptake and ryanodine receptor analysis. When expressed per mg of proteins, Ca uptake was decreased by 20% in failing hearts (FH) and ryanodine receptor density was similar in control hearts (CH) and in FH. However Ca uptake and ryanodine receptor density were significantly increased when expressed per total left ventricle suggesting SR hypertrophy. On electron microscopic examination, SR morphology not directly examined but large hypertrophied T tubules were observed suggesting a change in the relationship between membranes and contractile apparatus which may lead to alterations in excitation-contraction-relaxation coupling in spite of minimal biochemical alterations of SR.

Expression of calcium channels in Xenopus oocyte injected with crab skeletal muscle fibre mRNA.

Using the whole cell voltage-clamp technique and a C1 free and Na free Ba methane sulfonate solution, stage V and VI Xenopus oocytes demonstrated a Ba current (endogenous component) with a peak amplitude average of 6 nA (6 +/- 2 nA). When oocytes were injected with crustacean skeletal muscle mRNA, an additional component of IBa could be detected (exogenous IBa). The latter current could be distinguished from the native one by several electrophysiological means: a peak amplitude average of 90 nA (90 +/- 4 nA), activation potential threshold, steady state inactivation properties and sensitivity to Ca blockers. As shown by Jdaïâa and Guilbault in crustacean skeletal muscle fibres, exogenous IBa could be divided into two components: a "fast component" and a "slow component" probably passing through two types of Ca channels (fast and slow) since the peak Ba current voltage relationship was biphasic and the fast component of exogenous IBa was less sensitive than the slow to nifedipine. The features of the newly synthesized channels incorporated in the Xenopus oocyte membrane suggest that they may be associated with fast and slow channels, previously described in many preparations, particularly in crustacean skeletal muscle fibres.

Possible effects of membrane polarization on calcium uptake by and release from sarcoplasmic reticulum vesicles.

Rates of calcium uptake by and calcium release from sarcoplasmic reticulum vesicles isolated from skeletal muscle of the crab seem to depend on membrane potential generated by potassium (K) and chloride (Cl) gradients. This does not appear to be due to an effect of the ions themselves since media of different ionic compositions leading to the same membrane potential, also lead to the same ATP hydrolysis and the same Ca uptake by SR vesicles. From a large positive intravesicular potential (conditions termed "normal" in this paper), membrane depolarization of passively Ca loaded vesicles, produced by changes in K and Cl concentrations in the media, resulted in: i) decrease in rate of calcium uptake; ii) decrease in calcium loading; iii) increase in rate of calcium release despite a decrease in the driving force for calcium ions. Moreover, the addition of caffeine (5 mmol/l) to the different polarization media resulted in a increase in calcium release.

Is inward calcium current in crayfish muscle membrane constituted of one or two components?

Two inward currents were observed in crayfish muscle membrane during depolarization steps by the method described by Adrian et al. (1970). Under voltage clamp conditions, hyperpolarization steps elicited a large current (leak current If), associated with an inward voltage dependent current. This inward current was inhibited by niflumic acid (NA), a drug known to block Cl---HCO-3 exchange (Cousin et Motais 1982; Brûlè et al. 1983b). Dynamic outward currents triggered by depolarizing steps were inhibited to a great extent by TEA, the not inhibited portion disappearing when procaine (2 mmol/l) was added to external solution. In the presence of TEA, procaine and NA, it was thus possible to dissect the regenerative calcium current (ICa) into two components: a "fast component" (ICa1) and a "slow component" (ICa2). The reversal potential of ICa was 65 mV (for [Ca]0 = 2.8 mmol/l), and [Ca]i could be calculated to be 1.6 X 10(-5) mol/l. This value of [Ca]i is the same as calculated from values reported by Hencek and Zachar (1977). ICa1 was triggered at a threshold membrane potential of -45 mV and ICa2 at -30 mV. Moreover, the inactivation kinetics for ICa1 was faster than that for ICa2. Our results are in perfect agreement with those obtained by Zahradník and Zachar (1982) who postulated two populations of calcium channels.

[Inhibition, by an amphiphilic substance, niflumic acid, of the inward rectification of the crustacean muscle fiber]. / Inhibition par une substance amphiphile, l'acide niflumique, de la rectification dans le sens entrant de la fibre musculaire de Crustacé.

Agents such as TEA+ or CS+ ions, these last ions instead of K+ ions in poor K extracellular solution, known to reduce or abolish the inwardly rectifying channel in many preparations produced no effect in crayfish muscle membrane By contrast, poor Cl extracellular solution (Cl- ions were replaced by CH3OSO3- ions) blocked the inward current activated by hyperpolarizing pulses and produced an increase of the resting potential. Niflumic acid is a agent which inhibited the inward going rectification of the crayfish muscle membrane. Apparent dissociation constant of niflumic acid with membrane sites was equal to about 6 X 10(-8) M; this value corresponds to that given by Cousin & Motais (1979) concerning translocation of Cl- ions in the membrane of red cells. Activation of the inward going rectification in the crayfish membrane is responsible of an inward current carried by Cl- ions.