Depletion Of Extracellular Ca Research Articles

Effects of acetylcholine, TSH and other stimulators on intracellular calcium concentration in dog thyroid cells

The intracellular free calcium concentration, [Ca 2+] i, has been measured in dog thyroid cells using the fluorescent Ca 2+-indicator, quin2. Acetylcholine or its non-hydrolyzable analog, carbamylcholine rapidly increased [Ca 2+] i by 40 ± 4% (mean ± SE) over the basal level of 81 ± 2 nM. This increase was totally abolished by atropine, a muscarinic cholinergic receptor blocker, but was not influenced by verapamil, a voltage dependent-calcium channel blocker. Depletion of extracellular Ca 2+ by the addition of EGTA, diminished but did not abolish the response to carbamylcholine. These data suggest that cholinergic effectors increase [Ca 2+] i by mobilization of Ca 2+ from intracellular stores rather than from an influx of Ca 2+. Addition of TSH, isoproterenol, phorbol ester, dibutyryl cyclic GMP or cyclic AMP did not elicit any change in [Ca 2+] i suggesting that their action may not involve any mobilization of intracellular Ca 2+. These data provide direct evidence that in the thyroid cell, cholinergic agents act via their receptors to cause a rapid increase in [Ca 2+] i, which may mediate their metabolic effects.

Early transient depletion of extracellular Ca during individual cardiac muscle contractions.

Extracellular free [Ca] in rabbit papillary muscles was monitored using double-barreled Ca microelectrodes. These electrodes had tip diameters of 4-12 microns, electrical time constants of 2-5 ms, and electrochemical time constants of less than 30 ms. During individual beats a transient depletion of extracellular Ca (CaO) was recorded. This decrease of [Ca]O begins very early during the action potential, before significant tension development, and reaches a maximum much before the peak of developed tension (T). The depletion of CaO is blocked by CoCl2 or verapamil and enhanced by 10(-8) M isoproterenol or reduction of extracellular Na concentration to 35 mM. The magnitude of this early depletion of CaO increases in parallel with tension as a function of [Ca]O (8.46 +/- 0.98 microM at 0.2 mM CaO, 16.9 +/- 1.6 microM at 0.5 mM CaO, and 44.7 +/- 3.7 microM at 2.0 mM CaO). However the magnitude (in mV) of the recorded signal decreases with increasing [Ca]O and T, suggestive of saturation. The magnitude of this early transient CaO depletion also increases in parallel with the increase of T produced by initiating stimulation from rest (except for the first beat, which may be more dependent on stored Ca). It is probable that the depletions recorded represent Ca influx into cardiac cells from the extracellular space. The magnitude of Ca influx represented by the CaO depletions is difficult to quantitate but may be roughly in the range of Ca entry that would be required for direct activation of the myofilaments.

TIME- AND Na-DEPENDENT EFFECTS OF Ca DEPLETION ON POTASSIUM CONTRACTURE IN FROG TWITCH MUSCLE FIBER

The effect of the extracellular Ca depletion on potassium contracture was investigated in single fibers isolated from frog semitendinosus muscle mainly in relation to its time and Na dependency. The shortening of plateau duration and the increase in the rate of relaxation of the potassium contracture appeared within 3--5 sec and 15 sec, respectively, after the fiber was immersed in Ca-free Na Ringer solution containing 1 mM ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) (EGTA-Na Ringer solution) or Ca-free choline Ringer solution containing 1 mM (EGTA (EGTA-choline Ringer solution). These effects were independent of the presence or absence of extracellular Na. In EGTA-Na Ringer solution, the potassium contracture tension was inhibited only by about 20% after 20--90 min and was abolished after 120 min. The inhibition of the peak tension was accelerated by the depletion of extracellular Na; in EGTA-choline Ringer solution, the tension was gradually inhibited by about 20% during the first 7 min and abolished after 10--12 min. When the peak tension of potassium contracture was abolished in EGTA-choline Ringer solution, the depolarization by Ca depletion was about 10 mV and the caffeine contracture was sufficiently produced. The results suggest that the inhibition of the potassium contracture tension in EGTA-choline Ringer solution is due to the dissociation of excitation-contraction coupling. On the basis of these results, an aspect of the inactivation of the potassium contracture was proposed.