Rise In Intracellular Ca Concentration Research Articles

Characteristics of cytosolic Ca 2+ elevation induced by muscarinic receptor activation in single adrenal chromaffin cells of the guinea pig

In Fura-2 loaded-single guinea pig adrenal chromaffin cells, muscarine, nicotine and KCl all caused an early peak rise in intracellular Ca concentration ([Ca 2+] i) followed by a sustained rise. In Ca 2+-free solution, muscarine, but neither nicotine nor KCl, caused a transient increase in [Ca 2+] i, which was partially reduced by preceding application of caffeine or by treatment with ryanodine plus caffeine. In voltage-clamped cells at a holding potential of −60 mV, the muscarine-induced [Ca 2+] i, rise, especially its sustained phase, decreased in magnitude. intracellular application of inositol 1,4,5-trisphosphate caused a transient increase in [Ca 2+] i and inhibited the following [Ca 2+] i response to muscarine without affecting responses to nicotine and a depolarizing pulse. Muscarine evoked membrane depolarization following brief hyperpolarization in most cells tested. There was a significant positive correlation between the amplitude of the depolarization and the magnitude of the sustained rise in [Ca 2+] i. Muscarine-induced sustained [Ca 2+] i rise was much greater in the current-clamp mode than that in the voltage-clamp mode. The sustained phase of [Ca 2+] i rise and Mn 2+ influx in response to muscarine were suppressed by a voltage-dependent Ca 2+ channel blocker, methoxyverapamil. These results suggest that stimulation of muscarinic receptors causes not only extracellular Ca 2+ entry, but also Ca 2+ mobilization from inositol 1,4,5-trisphosphate-sensitive intracellular stores. Voltage-dependent Ca 2+ channels may function as one of the Ca 2+ entry pathways activated by muscarinic receptor in guinea pig adrenal chromaffin cells.

Reoxygenation-induced arrhythmogenic transient inward currents in isolated cells of the guinea-pig heart

Transient inward currents (Iti), activated by a rise in intracellular Ca concentration, are believed to trigger cardiac arrhythmias in reperfused hearts. In this report, Iti in isolated cardiocytes from the guinea-pig were evoked by reoxygenation following a period of anoxia of between 4 min and 35 min. Reoxygenation was performed 1 min after the full development of an anoxia-induced time-independent K current. This current disappeared within 2-6 s and in the following 10 s Iti developed to maximum amplitude. Iti were evoked using a constant pulse pattern (holding potential Vh = -45 mV; test potential Vt = +10; pulse duration 350 ms; frequency 1 Hz). In more than 95% of the cells, Iti at the holding potential Iti (-45 mV) declined with a time constant of tau = 670 +/- 240 ms (mean +/- SD, n = 17). In two cells, undamped oscillatory currents were observed. The amplitude of Iti (-45 mV) was proportional to the amplitude and duration of the preceding depolarizing test pulse. Test pulses of long duration (500 ms and 1000 ms, mean +/- SD) to potentials positive to +10 mV produced slowly decaying tail currents (tau = 391 +/- 51 ms, mean +/- SD), which superimposed with Iti (-45 mV). The current/voltage relationship of Iti peaked between -30 mV and -10 mV and approximated zero at the most positive potentials, i.e. no reversal of Iti was found up to +80 mV. Using double-pulse protocols (prepulse potential +40 mV), Iti were enhanced at potentials negative to -30 mV and were also present in the range of the normal resting potential of ventricular heart cells. The instantaneous current-voltage relationship was monotone between -50 mV and +40 mV. Because of the dependence of Iti on the preceding depolarization, the instantaneous current-voltage relationship provides more reliable information on the voltage dependence of Iti. The interval between two subsequent Iti (-45 mV) values was 237 +/- 35 ms (mean +/- SD, n = 27) and depended on the amplitude of Iti (-45 mV) to increase by 5.2 +/- 0.5% (mean +/- SD) per 100 pA decrease in Iti (-45 mV). A simple noise analysis showed that if one assumes that ionic channels are responsible for the generation of Iti (-45 mV), their unitary conductance cannot exceed 0.36 pS. We conclude that reoxygenation-induced Iti are triggered by a cyclic release of Ca from the sarcoplasmic reticulum and provide evidence that they are mediated by the electrogenic Na/Ca exchanger.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium-activated potassium channels in isolated presynaptic nerve terminals from rat brain.

86Rb efflux was examined in isolated presynaptic nerve terminals (synaptosomes) from rat brain in a study designed to assess K permeability (PK) changes sensitive to alterations in internal Ca activity. Rb efflux from 86Rb-loaded synaptosomes into nominally Ca-free physiological saline (PSS) containing 5 mM-K was about 0.3-0.4%/s. Raising extracellular K concentration [( K]o), to depolarize the synaptosomes, stimulated the 86Rb efflux. Addition of Ca to the 5 mM-K PSS had no effect, but Ca did further stimulate 86Rb efflux into K-rich solutions. The effect of Ca was graded, with apparent half-maximal activation, KA approximately equal to 0.5 mM-Ca. These data fit the view that, during depolarization, Ca enters the terminals through voltage-regulated Ca channels, and that the rise in intracellular Ca concentration opens certain (Ca-activated) K channels. The Ca-dependent stimulation of 86Rb efflux was greatest during the initial seconds of incubation (component CT), and then declined to a much lower rate (component CS). Much of this change in rate could be attributed to inactivation of voltage-regulated Ca channels and reduced entry of Ca. The Ca-dependent increase in 86Rb efflux was completely inhibited by 100 microM-La. In the presence of Ca, but not in its absence, the Ca ionophore A23187 stimulated 86Rb efflux both in 5 and 100 mM-K PSS. The effect in 100 mM-K was quantitatively greater, perhaps because of the increased outward driving force on Rb in depolarized synaptosomes. When synaptosomes were suspended in media containing the voltage-sensitive fluorescent dye, DiS-C3-(5) (1,1'-dipentyl-2,2'-thiocarbocyanine), the addition of Ca+ A23187 decreased the fluorescence intensity (= synaptosome hyperpolarization) when the media contained 5 mM-K but not 100 mM-K. This implies that in the presence of Ca + A23187, PK was increased, and the membrane potential moved closer to the K equilibrium potential, EK. Quinine sulphate, a blocker of Ca-activated K channels, reduced the Ca-stimulated 86Rb efflux with high affinity (apparent half-maximal inhibition, KI approximately equal to 1 microM). Tetraethylammonium chloride, another agent known to block Ca-activated K channels, was also a relatively potent inhibitor of Ca-stimulated 86Rb efflux (KI approximately equal to 0.2 mM). The K-channel blocker, 4-aminopyridine, partially inhibited Ca-stimulated 86Rb efflux at concentrations below 0.5 mM, but stimulated this efflux at concentrations greater than or equal to 1 mM.(ABSTRACT TRUNCATED AT 400 WORDS)

The depolarizing afterpotential of crab muscle fibres. A sodium-dependent process mediated by intracellular calcium.

1. A study was made of the depolarizing afterpotential (d.a.p.) which follows the initial graded electrogenesis of crab muscle fibres. 2. Increasing the strength, duration or amplitude of the stimulating current pulses enhanced both the d.a.p. and the local contractions. 3. Arsenazo III was injected intracellularly and changes in light absorbance by the dye were used to monitor the increase in free sarcoplasmic Ca concentration during excitation-contraction (E-C) coupling. The onset of the absorbance changes occurred during the depolarizing phase of the initial electrogenesis and the maximum value coincided with the peak of the d.a.p. An exponential decay of the absorbance signal occurred during the repolarizing phase of the d.a.p. 4. Ionophoretic injection of EGTA into the sarcoplasm did not affect the initial electrogenesis but did reduce changes in dye absorbance, blocked tension development and abolished the d.a.p. 5. Caffeine (0.1--0.4 mM) markedly enhanced both the d.a.p. and the local contractions, but had no effect on the initial electrogenesis. 6. Replacement of extracellular Na ions with Li, Tris or choline abolished the d.a.p. The initial electrogenesis was enhanced in the choline-containing medium, but was not affected by Li or Tris. The rate of relaxation of the local contractions and the rate of decay of the light absorbance changes were slowed in Na-free saline. 7. Tetrodotoxin (10(-5) g/ml.) had no effect on either the membrane responses or tension development. 8. For initial graded responses of comparable peak amplitude a threefold reduction of [Ca-a1o shortened the d.a.p., but had little effect onits peak amplitude. A fivefold increase in [Ca]o reduced both the amplitude and duration of the d.a.p. 9. Changes in [Mg]o had little effect on the d.a.p., but both Mn (4--10 mM) and La (0.1 mM) blocked the initial electrogenesis and the d.a.p. 10. It is concluded that distinct ionic mechanisms give rise to the initial electrogenesis and the d.a.p. While the former is due to activation of Ca conductance, the d.a.p. is a Na-dependent phenomenon that is tetrodotoxin-insensitive, and mediated by the rise of intracellular Ca concentration during E--C coupling.