Evidence for localized calcium mobilization and influx in single rat gonadotropes

Secretory granule exocytosis

Regulated exocytosis of secretory granules or dense-core granules has been examined in many well-characterized cell types including neurons, neuroendocrine, endocrine, exocrine, and hemopoietic cells and also in other less well-studied cell types. Secretory granule exocytosis occurs through mechanisms with many aspects in common with synaptic vesicle exocytosis and most likely uses the same basic protein components. Despite the widespread expression and conservation of a core exocytotic machinery, many variations occur in the control of secretory granule exocytosis that are related to the specialized physiological role of particular cell types. In this review we describe the wide range of cell types in which regulated secretory granule exocytosis occurs and assess the evidence for the expression of the conserved fusion machinery in these cells. The signals that trigger and regulate exocytosis are reviewed. Aspects of the control of exocytosis that are specific for secretory granules compared with synaptic vesicles or for particular cell types are described and compared to define the range of accessory control mechanisms that exert their effects on the core exocytotic machinery.

Two endogenous gonadotropin-releasing hormones generate dissimilar Ca(2+) signals in identified goldfish gonadotropes

Ca(2+) signals are involved in the signal transduction of neuroendocrine regulators. In goldfish, two endogenous gonadotropin-releasing hormones, salmon (s)GnRH and chicken (c)GnRH-II, control maturational gonadotropin secretion. Although considerable evidence suggests that sGnRH and cGnRH-II exert their activity on goldfish gonadotropes through a single population of receptors, differences in signal transduction mechanisms between these peptides have been demonstrated. We used ratiometric Fura-2 Ca(2+) imaging of single morphologically identified gonadotropes to quantitatively compare the Ca(2+) signals evoked by sGnRH and cGnRH-II. The amplitude and the rate of rise of sGnRH- and cGnRH-II-evoked Ca(2+) signals increased with concentration. At maximal concentrations, Ca(2+) signals generated by cGnRH-II rose significantly faster than those elicited by sGnRH, while other parameters such as the maximum amplitude, average Ca(2+) increase, and latency did not differ between the two peptides. Ca(2+) signals evoked by sGnRH or cGnRH-II were often spatially restricted to one part of the cell over the duration of the response. We provide a comprehensive account of the spatial and temporal aspects, including calculated kinetics, of GnRH-evoked Ca(2+) signals in single identified gonadotropes. This is the first report of quantified differences in Ca(2+) signals generated by two endogenous GnRH neuropeptides, which may act through the same receptor population in this cell type.

From ultradian to infradian rhythms: LH release patterns in vitro

In the present study, we examined in vitro luteinizing hormone (LH) release patterns from pituitaries and from pituitary cell cultures (3 and 7 days in culture) to elucidate the endogenous period generated by the gonadotroph cell population and to evaluate the relationship between the basic period generated at the cellular level and the output pattern observed at the organ level. In addition, we examined the effect of photic environmental signals perceived by the animals on LH release patterns from pituitaries in vitro. When the animals were exposed to circadian photoperiodic signals, the in vitro LH release pattern from the pituitaries exhibited ultradian, circadian, and infradian frequencies. When the animals were exposed to continuous illumination, the in vitro patterns exhibited only ultradian and infradian frequencies. Furthermore, free running is a process, not a state. This process is driven by a change in the relative dominance of different frequencies that construct the pattern without changing the basic period length. Evaluation of the relative dominance of the different frequencies that construct the pattern indicates that, although infradian oscillators may take part in shaping the output pattern, the basic rhythm generated by the pituitary cells is in the ultradian domain. The results obtained from the examined system suggest that an endogenous oscillator is a cellular entity with ultradian periodicity, and that the rhythmic output of many biological variables is structured by various ultradian components that construct the circadian and infradian output rhythms.

Local Ca2+ release from internal stores controls exocytosis in pituitary gonadotrophs

Exocytosis and the cell-averaged cytosolic [Ca2+], [Ca2+]i, were tracked in single gonadotrophs. Cells released 100 granules/s at 1 microM = [Ca2+]i when gonadotropin-releasing hormone (GnRH) activated IP3-mediated Ca2+ release from internal stores, but only 1 granule/s when [Ca2+]i was raised uniformly to 1 microM by other means. Strong exocytosis was then seen only at higher [Ca2+]i (half-maximal at 16 microM). Parallel second messengers did not contribute to GnRH-induced exocytosis, because IP3 alone was as effective as GnRH, and because even GnRH failed to trigger rapid exocytosis when the [Ca2+]i rise was blunted by EGTA. When [Ca2+]i was released from stores, exocytosis depended on [Ca2+]i rising rapidly, as if governed by Ca2+ flux into the cytosol. We suggest that IP3 releases Ca2+ selectively from subsurface cisternae, raising [Ca2+] near exocytic sites 5-fold above the cell average.

Functional heterogeneity of pituitary gonadotropes in response to a variety of neuromodulators

Agents previously implicated in control of the hypothalamo-pituitary-gonadal axis were screened for their ability to regulate male rat gonadotropes directly. GnRH-evoked gonadotropin release is accompanied by oscillations of intracellular Ca2+ concentration ([Ca2+]i) and of an outward K+ current that is activated by Ca2+. Substances that caused current responses similar to those with GnRH were hypothesized to evoke secretion. Endothelin-1, oxytocin, neurotensin, pituitary adenylate cyclase-activating polypeptide, and serotonin raised [Ca2+]i and evoked LH release as assayed by the reverse hemolytic plaque assay. These agents affected only subpopulations of gonadotropes establishing functional heterogeneity of pituitary gonadotropes. One neuromodulator (ATP) evoked ionic current in all gonadotropes but the current was different than that evoked by GnRH. Many other substances, including galanin and neuropeptide Y, caused no changes in currents and were considered not to affect [Ca2+]i and not to be secretagogues for gonadotropes.

Spontaneous transients of [Ca2+]i depend on external calcium and the activation of L-type voltage-gated calcium channels in a clonal pituitary cell line (AtT-20) of cultured mouse corticotropes

Spontaneous transients of [Ca2+]i were recorded from single nonstimulated cells of a clonal pituitary cell line of corticotropes, AtT-20/D16v. The spontaneous [Ca2+]i transients were dependent on calcium entry from the extracellular solution because they were abolished both in the absence of extracellular calcium and with the addition of cobalt to the calcium-containing extracellular solution. Calcium entry occurred through voltage-gated (VGCC) L-type calcium channels because the [Ca2+]i transients were blocked by L-type calcium channel antagonists, e.g. nifedipine, and were unaffected by the addition of tetrodotoxin. Bay K 8644 (1 microM) induced transient increases in [Ca2+]i which were also blocked reversibly by either the absence of extracellular calcium or the addition of an L-type calcium channel antagonist (e.g. nifedipine). The resting levels of [Ca2+]i and the frequency, but not the amplitude or duration, of the spontaneous [Ca2+]i transients increased as the concentration of extracellular calcium was elevated in concentrations ranging from 1.8-7.2 mM. Potassium depolarization reversibly elevated resting levels of [Ca2+]i and initiated the spontaneous calcium transients. These results indicate that extracellular calcium modulates the frequency of spontaneous [Ca2+]i transients in AtT-20 cells which are caused by the activation of L-type calcium channels by a spontaneous increase in the permeability of the cell membrane to calcium.

Spatial localization of agonist-induced Ca2+ entry in bovine adrenal chromaffin cells. Different patterns induced by histamine and angiotensin II, and relationship to catecholamine release

The spatial organization of agonist-induced Ca2+ entry in single bovine adrenal chromaffin cells has been investigated using video-imaging techniques to visualize fura-2 quenching by the Ca2+ surrogate, Mn2+. The potent secretagogue histamine, in addition to releasing Ca2+ from intracellular stores, resulted in a large influx of external Mn2+ that occurred over the entire surface of the cell. The influx of Ca2+ that this mirrors was found to be an obligatory requirement for the triggering of catecholamine release by histamine, which suggests that such a global influx of Ca2+ into the cell probably underlies the ability of this agonist to stimulate a large secretory response. By contrast, the weaker secretagogue angiotensin II, which also acts through the second messenger inositol trisphosphate, produced a localized entry of external Mn2+ in 64% of cells. In these cells, localized Mn2+ entry always occurred at the pole of the cell in which the angiotensin II-induced rise in [Ca2+]i was largest. Since exocytosis in response to angiotensin II has previously been shown to be restricted to this same pole of the cell (Cheek et al. (1989). J. Cell Biol. 109, 1219–1227), these results suggest that localized influx of Ca2+ in response to angiotensin II could underlie the polarized exocytotic response observed with this stimulus. These results directly demonstrate that different agonists can induce different patterns of divalent cation influx in the same cells and, furthermore, suggest how these different patterns can have a direct influence on cellular function.