Regulation of gap junctional coupling in isolated pancreatic acinar cell pairs by cholecystokinin-octapeptide, vasoactive intestinal peptide (VIP) and a VIP-antagonist

Gap junction-mediated intercellular communication in the adrenal medulla: an additional ingredient of stimulus-secretion coupling regulation

The traditional understanding of stimulus-secretion coupling in adrenal neuroendocrine chromaffin cells states that catecholamines are released upon trans-synaptic sympathetic stimulation mediated by acetylcholine released from the splanchnic nerve terminals. Although this statement remains largely true, it deserves to be tempered. In addition to its neurogenic control, catecholamine secretion also depends on a local gap junction-mediated communication between chromaffin cells. We review here the insights gained since the first description of gap junctions in the adrenal medullary tissue. Adrenal stimulus-secretion coupling now appears far more intricate than was previously envisioned and its deciphering represents a challenge for neurobiologists engaged in the study of the regulation of neuroendocrine secretion. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Revisiting the stimulus-secretion coupling in the adrenal medulla: role of gap junction-mediated intercellular communication

The current view of stimulation-secretion coupling in adrenal neuroendocrine chromaffin cells holds that catecholamines are released upon transsynaptic sympathetic stimulation mediated by acetylcholine released from the splanchnic nerve terminals. However, this traditional vertical scheme would merit to be revisited in the light of recent data. Although electrical discharges invading the splanchnic nerve endings are the major physiological stimulus to trigger catecholamine release in vivo, growing evidence indicates that intercellular chromaffin cell communication mediated by gap junctions represents an additional route by which biological signals (electrical activity, changes in intracellular Ca(2+) concentration,...) propagate between adjacent cells and trigger subsequent catecholamine exocytosis. Accordingly, it has been proposed that gap junctional communication efficiently helps synapses to lead chromaffin cell function and, in particular, hormone secretion. The experimental clues supporting this hypothesis are presented and discussed with regards to both interaction with the excitatory cholinergic synaptic transmission and physiopathology of the adrenal medulla.

Pharmacology of gap junctions. New pharmacological targets for treatment of arrhythmia, seizure and cancer?

Intercellular communication in many organs is maintained via intercellular gap junction channels composed of connexins, a large protein family with a number of isoforms. This gap junction intercellular communication (GJIC) allows the propagation of action potentials (e.g., in brain, heart), and the transfer of small molecules which may regulate cell growth, differentiation and function. The latter has been shown to be involved in cancer growth: reduced GJIC often is associated with increased tumor growth or with de-differentiation processes. Disturbances of GJIC in the heart can cause arrhythmia, while in brain electrical activity during seizures seems to be propagated via gap junction channels. Many diseases or pathophysiological conditions seem to be associated with alterations of gap junction protein expression. Thus, depending on the target disease opening or closure of gap junctions may be of interest, or alteration of connexin expression. GJIC can be affected acutely by changing gap junction conductance or--more chronic--by altering connexin expression and membrane localisation. This review gives an overview on drugs affecting GJIC.

Regulation of ion fluxes, cell volume and gap junctional coupling by cGMP in GFSHR-17 granulosa cells

Gap junctional communication between granulosa cells seems to play a crucial role for follicular growth and atresia. Application of the double whole-cell patch-clamp- and ratiometric fura-2-techniques allowed a simultaneous measurement of gap junctional conductance ( G(j)) and cytoplasmic concentration of free Ca(2+) ([Ca(2+)](i)) in a rat granulosa cell line GFSHR-17. The voltage-dependent gating of G(j) varied for different cell pairs. One population exhibited a bell-shape dependence of G(j) on transjunctional voltage, which was strikingly similar to that of Cx43/Cx43 homotypic gap junction channels expressed in pairs of oocytes of Xenopus laevis. Within 15-20 min, gap junctional uncoupling occurred spontaneously, which was preceded by a sustained increase of [Ca(2+)](i) and accompanied by shrinkage of cellular volume. These responses to the whole-cell configuration were avoided by absence of extracellular Ca(2+), blockage of K(+) efflux, or addition of 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP) to the pipette solution. Even in the absence of extracellular Ca(2+) or blockage of K(+) efflux, formation of whole-cell configuration generated a Ca(2+) spike that could be suppressed by the presence of 8-Br-cGMP. We propose that intracellular cGMP regulates Ca(2+) release from intracellular Ca(2+) stores, which activates sustained Ca(2+) influx, K(+) efflux and cellular shrinkage. We discuss whether gap junctional conductance is directly affected by cGMP or by cellular shrinkage and whether gap junctional coupling and/or cell shrinkage is involved in the regulation of apoptotic/necrotic processes in granulosa cells.

Calmodulin antagonists suppress gap junction coupling in isolated Hensen cells of the guinea pig cochlea

The effect of calmodulin (CaM) antagonists W7, trifluoperazine (TFP) and a calmodulin inhibitory peptide on gap junction coupling in isolated Hensen cells of the organ of Corti was analysed by the double whole-cell patch-clamp technique. Addition of the conventional antagonists W7 and TFP in the micromolar range caused a rapid decrease of gap junction conductance after a delay of a few minutes in a dose-dependent manner. Fluorescence spectroscopy of cytoplasmic free calcium concentration ([Ca(2+)](i)) by Fura-2 showed no significant change of [Ca(2+)](i) by W7. Chelation of [Ca(2+)](i) by 10 mM BAPTA or use of nominally Ca(2+)-free external bath did not suppress the W7-induced gap junction uncoupling. The results suggest that W7 and TFP induce gap junction uncoupling at unchanged global [Ca(2+)](i) in Hensen cells. To obtain additional evidence for an involvement of CaM in regulating gap junction conductance a calmodulin inhibitory peptide, the MLCK peptide (250 nM), was added to the standard pipette solution. Again gap junction uncoupling was observed, but on a significantly slower time scale. This is the first study of an effect of calmodulin antagonists on gap junction coupling in isolated Hensen cells. The question whether the effect of calmodulin inhibitors is specific and involves CaM-dependent gating of gap junction coupling in Hensen cells is discussed.

Cholecystokinin-octapeptide affects the fluorescence signal of a single pancreatic acinar cell loaded with the acrylodan-labelled MARCKS peptide, a protein kinase C substrate

We used a fluorescent derivative of the myristoylated, alanine-rich C kinase substrate (MARCKS) peptide as a probe for protein kinase C (PKC) activation by cholecystokinin-octapeptide (CCK-8) in isolated pancreatic acinar cell pairs. The diffusion of the acrylodan-labelled MARCKS-peptide into the cell interior could be monitored by the increase of fluorescence in the whole-cell patch-clamp configuration. Addition of 10 pM CCK-8 to the bath induced repetitive fluctuations of the fluorescent signal in the time range of 4-5 min. With 1 nM CCK-8 a sustained decrease of the signal was observed. Addition of polymyxin B, a specific inhibitor of PKC activation, to the pipette filling solution suppressed the CCK-8-induced change of fluorescence. The data indicate activation of PKC by CCK-8 in pancreatic acinar cells and could be compared with the previously studied CCK-8-induced gap junction uncoupling.