Ca(2+)- and voltage-dependent inactivation of Ca2+ currents in rat intermediate pituitary

Action currents generate stepwise intracellular Ca2+ patterns in a neuroendocrine cell

It is believed that specific patterns of changes in the cytosolic-free calcium concentration ([Ca2+]i) are used to control cellular processes such as gene transcription, cell proliferation, differentiation, and secretion. We recently showed that the Ca2+ oscillations in the neuroendocrine melanotrope cells of Xenopus laevis are built up by a number of discrete Ca2+ rises, the Ca2+ steps. The origin of the Ca2+ steps and their role in the generation of long-lasting Ca2+ patterns were unclear. By simultaneous, noninvasive measuring of melanotrope plasma membrane electrical activity and the [Ca2+]i, we show that numbers, amplitude, and frequency of Ca2+ steps are variable among individual oscillations and are determined by the firing pattern and shape of the action currents. The general Na+ channel blocker tetrodotoxin had no effect on either action currents or the [Ca2+]i. Under Na+-free conditions, a depolarizing pulse of 20 mM K+ induced repetitive action currents and stepwise increases in the [Ca2+]i. The Ca2+ channel blocker CoCl2 eliminated action currents and stepwise increases in the [Ca2+]i in both the absence and presence of high K+. We furthermore demonstrate that the speed of Ca2+ removal from the cytoplasm depends on the [Ca2+]i, also between Ca2+ steps during the rising phase of an oscillation. It is concluded that Ca2+ channels, and not Na+ channels, are essential for the generation of specific step patterns and, furthermore, that the frequency and shape of Ca2+ action currents in combination with the Ca2+ removal rate determine the oscillatory pattern.

Kinetics of calcium steps underlying calcium oscillations in melanotrope cells of Xenopus laevis

Melanotrope cells of Xenopus laevis display intracellular calcium oscillations which are generated at the plasma membrane and travel as a wave through the cytoplasm into the nucleus. An oscillation involves discrete increases in intracellular Ca2+ ('steps'), followed by a relatively smooth return to the basal Ca2+ level. The aim of our investigation was to determine what role these steps play in shaping the Ca2+ signal in melanotrope cells, by conducting a high resolution spatio-temporal analysis of the kinetics of the Ca2+ steps. To this end Fura-red loaded cells were analysed by confocal laser scanning microscopy using the line scanning method to achieve 6 ms time resolution. Furthermore, the kinetics of the steps were analysed in 3 different intracellular areas, to see if there are spatial differences in Ca2+ signalling kinetics. The results showed that each calcium oscillation is built up by 3-4 steps that were generated very quickly and had approximately the same size. Following each Ca2+ step, there was a slow removal of calcium before the next step boosted the overall level of Ca2+. Since the Ca2+ steps were most pronounced directly beneath the plasma membrane, they appear to be generated in this region. The speed of the Ca2+ wave near the membrane exceeded 40 microns/s, indicating an active mechanism for wave propagation. In deeper regions of the cell, the wave speed was much slower (about 8 microns/s) and the size of each step was smaller, indicating that regulation occurs within a narrower range of [Ca2+]i. Inside the nucleus, however, the calcium wave accelerated again (23 microns/s). Treatment with TRH evoked a high amplitude Ca2+ transient and increased the number of Ca2+ steps to 5 or 6. Each step had approximately the same size as the steps of the pretreatment Ca2+ oscillations. Caffeine treatment, which increased the frequency of the oscillations, had no effect on the number or the size of the Ca2+ steps, but it reduced the time needed for each step to reach its maximum height. We suggest a possible 'building block' function for the Ca2+ steps, whereby a cell generates more steps to achieve a high oscillation amplitude or accelerates the speed of the steps to increase the frequency of oscillations. Both phenomena may play a crucial role in the encoding of information transduced from an extracellular input to the intracellular target.

Voltage-clamp analysis of the voltage-gated sodium current of the rat pituitary melanotroph

By using the whole-cell recording technique the Na+ current in cultured melanotrophs of the adult rat pituitary was studied. The Na+ current was eliminated by 1 microM TTX and its equilibrium potential confirmed that it was carried predominantly by Na+. The activation threshold was near -40 mV and half-maximal activation occurred at approximately -23 mV. The peak amplitude of 640 +/- 110 pA (n = 8) occurred near -10 mV. Steady-state half-inactivation occurred near -50 mV. Recovery from inactivation at -70 mV was biexponential: approximately half of the channels recovered with a time constant of 13 ms whereas the slower phase of recovery had a time constant of 430 ms. These properties of the Na+ current are discussed in relation to its role in cell firing and hormone secretion.

Spontaneous calcium oscillations in Xenopus laevis melanotrope cells are mediated by omega-conotoxin sensitive calcium channels

The dynamics of intracellular Ca2+ signalling in single melanotrope cells of the pituitary gland of the amphibian Xenopus laevis have been studied by means of a digital imaging technique using the fluorescent dye Fura-2. When placed in vitro, the majority of the cells (77%) displayed spontaneous oscillatory changes in the free cytosolic Ca2+ concentration with a frequency of 1 +/- 0.25 (SD) min-1. The oscillations rapidly stopped when extracellular Ca2+ was reduced to nanomolar concentrations, revealing their complete dependence on Ca2+ influx. The fact that the Ca2+ oscillations were blocked by 1 microM omega-conotoxin, but not by nifedipine, at concentrations up to 50 microM, indicated that Ca2+ entered the cell via N-type rather than L-type voltage operated Ca2+ channels. Thapsigargin, a putative inhibitor of intracellular Ca(2+)-ATPase activity, elevated the baseline Ca2+ concentration but had no effect on the occurrence of the spontaneous oscillations. This suggests that intracellular Ca2+ pools are not involved in the mechanism underlying spontaneous Ca2+ oscillations. This is the first report showing spontaneous Ca2+ oscillations mediated by N-type Ca2+ channels in melanotrope cells.

Blockade by funnel web toxin of a calcium current in the intermediate pituitary of the rat

The pharmacological sensitivities of the low threshold (LT) and high threshold (HT) calcium currents were studied using single electrode voltage clamp techniques in melanotrophs of the intact rat intermediate pituitary. The T-type LT current was selectively abolished by 200 microM nickel whereas the HT current was preferentially abolished by 25 microM cadmium. The HT current consisted of both sustained and inactivating components. The sustained portion of the HT current was increased by BAY K-8644 indicating the presence of an L-type current. The inactivating component of the HT current was not affected by omega-conotoxin, which blocks the N-type calcium current in many other cell types, but was rapidly and reversibly reduced by funnel-web toxin, a blocker of the P-type calcium channel. These data suggest the presence of a P-type channel in a neuroendocrine cell.

Ca(2+)-dependent inactivation of Ca2+ current in Aplysia neurons: kinetic studies using photolabile Ca2+ chelators

1. The kinetics and sensitivity of the Ca(2+)-dependent inactivation of calcium current (ICa) were examined in intact cell bodies from the abdominal ganglion of Aplysia californica under two-electrode voltage clamp. 2. Rapid changes in the level of intracellular free calcium ([Ca2+]i) were generated at the cell surface by photolytic release of Ca2+ (nitr-5 and dimethoxy nitrophen) or Ca2+ buffer (diazo-4). 3. Diazo-4 increased ICa by 10-15% and slowed the rate of ICa decay when photolysed before a test pulse or between a prepulse and a test pulse. The predominant effect of further light flashes was to increase the amount of non-inactivating current (I infinity) remaining at the end of long (> 1 s) depolarizing pulses. 4. A rapid increase in [Ca2+]i buffering during ICa inactivation did not cause a rapid recovery of current but merely reduced the rate and extent of subsequent inactivation. This effect was not seen when Ba2+ was the charge carrier. 5. Photolytic release of Ca2+ from nitr-5 produced estimated Ca2+ jumps of 3-4 microM at the front surface of the cell but failed to augment inactivation either before or during ICa. In contrast, photolysis of DM-nitrophen 10-90 ms before the test pulse decreased peak ICa by about 30%. A flash given during ICa rapidly blocked 41 +/- 3% of peak current with a time constant of 3-4 ms at 17 degrees C. Similar results were seen with the barium current (IBa). 6. Microinjection of the potent phosphatase inhibitor microcystin-LR (5 microM) had variable effects on ICa inactivation and augmented the cyclic AMP-induced depression of the delayed rectifier (IK(V) by forskolin (100 microM) and 3-isobutyl-1-methylxanthine (IBMX; 200 microM). 7. Full recovery from inactivation measured in two-pulse experiments took at least 20 s. This slow recovery process was unaffected by increases in intracellular cyclic AMP elicited by direct injection or by bath application of forskolin and IBMX. It was also unaffected by decreases in cyclic AMP induced by injecting 2',5'-dideoxyadenosine (1 mM) or bath application of the Rp isomer of cyclic adenosine 3',5'-monophosphothioate (Rp-cAMPS; 200 microM). 8. A 'shell' model relating submembrane Ca2+ to inactivation was inconsistent with the experimental results since it greatly overestimated the effects of diazo-4 and predicted significant inactivation by nitr-5 photolysis. 9. A model linearly relating [Ca2+]i in a single Ca2+ channel 'domain' to inactivation more closely matched the experimental results with diazo-4 and DM-(ABSTRACT TRUNCATED AT 400 WORDS)