Adenosine 3',5'-cyclic monophosphate stimulates secretion of alpha-melanocyte-stimulating hormone from permeabilized cells of the intermediate lobe of the rat pituitary gland

cAMP increases Ca2+-dependent exocytosis through both PKA and Epac2 in mouse melanotrophs from pituitary tissue slices

Cyclic AMP regulates Ca(2+)-dependent exocytosis through a classical protein kinase A (PKA)-dependent and an alternative cAMP-guanine nucleotide exchange factor (GEF)/Epac-dependent pathway in many secretory cells. Although increased cAMP is believed to double secretory output in isolated pituitary cells, the direct target(s) for cAMP action and a detailed and high-time resolved analysis of the effect of intracellular cAMP levels on the secretory activity in melanotrophs are still lacking. We investigated the effect of 200 microM cAMP on the kinetics of secretory vesicle depletion in mouse melanotrophs from fresh pituitary tissue slices. The whole-cell patch-clamp technique was used to depolarize melanotrophs and increase the cytosolic Ca(2+) concentration ([Ca(2+)](i)). Exogenous cAMP elicited an about twofold increase in cumulative membrane capacitance change and approximately 34% increase of high-voltage activated Ca(2+) channel amplitude. cAMP-dependent mechanisms did not affect [Ca(2+)](i), since the application of forskolin failed to change [Ca(2+)](i) in melanotrophs, a phenomenon readily observed in anterior lobe. Depolarization-induced secretion resulted in two distinct kinetic components: a linear and a threshold component, both stimulated by cAMP. The linear component (ATP-independent) probably represented the exocytosis of the release-ready vesicles, whereas the threshold component was assigned to the exocytosis of secretory vesicles that required ATP-dependent reaction(s) and > 800 nM [Ca(2+)](i). The linear component was modulated by 8-pCPT-2Me-cAMP (Epac agonist), while either H-89 (PKA inhibitor) or Rp-cAMPS (the competitive antagonist of cAMP binding to PKA) completely prevented the action of cAMP on the threshold component. In line with this, 6-Phe-cAMP, (PKA agonist), increased the threshold component. From our study, we suggest that the stimulation of cAMP production by application of oestrogen, as found in pregnant mice, increases the efficacy of the hormonal output through both PKA and cAMP-GEFII/Epac2-dependent mechanisms.

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.

Modulation of the unitary exocytic event amplitude by cAMP in rat melanotrophs

1. Secretory responses were measured in single rat pituitary melanotrophs as the relative increase in membrane capacitance (Cm) 8 min after the start of dialysis with solutions containing 0.45 microM Ca2+. In the added presence of cAMP (0.2 mM) in the patch pipette solution, capacitance responses increased 2- to 3-fold in comparison with controls. 2. To study whether cAMP-dependent mechanisms affect cytosolic calcium activity ([Ca2+]i), dibutyryl cyclic AMP (dbcAMP, 10 mM) was added to intact melanotrophs and [Ca2+]i was measured using fura-2 AM. Addition of dbcAMP caused a transient reduction in [Ca2+]i to 82 +/- 21 nM from a resting value of 100 +/- 19 nM (mean +/- S.E.M., n = 32, P < 0.002), indicating that the cAMP-induced increase in secretory activity was not the result of cAMP acting to increase [Ca2+]i, which then increased secretory activity. 3. To investigate whether cAMP affects the secretory apparatus directly, the interaction of a single secretory granule with the plasmalemma was monitored by measuring discrete femtofarad steps in Cm. The signal-to-noise ratio of recordings was increased by pre-incubating the cells with a hydrophobic anion, dipicrylamine. 4. Recordings of unitary exocytic events (discrete 'on' steps in Cm) showed that the amplitude of 'on' steps - a parameter correlated to the size of exocytosing secretory granules - increased from 4.2 +/- 0.2 fF (n = 356) in controls to 7.9 +/- 0.2 fF in the presence of cAMP (n = 329, P < 0.001), while the frequency of unitary exocytic events was similar in controls and in the presence of cAMP. 5. The results suggest that a cAMP-dependent mechanism mediates the fusion of larger granules with the plasmalemma.

Ca2+ and protein kinase C activation of mucin granule exocytosis in permeabilized SPOC1 cells

Mucin secretion by airway goblet cells is under the control of apical P2Y2, phospholipase C-coupled purinergic receptors. In SPOC1 cells, the mobilization of intracellular Ca2+ by ionomycin or the activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate (PMA) stimulates mucin secretion in a fully additive fashion [L. H. Abdullah, J. D. Conway, J. A. Cohn, and C. W. Davis. Am. J. Physiol. 273 (Lung Cell. Mol. Physiol. 17): L201-L210, 1997]. This apparent independence between PKC and Ca2+ in the stimulation of mucin secretion was tested in streptolysin O-permeabilized SPOC1 cells. These cells were fully competent to secrete mucin when Ca2+ was elevated from 100 nM to 3.1 microM for 2 min following permeabilization; the Ca2+ EC50 was 2.29 +/- 0.07 microM. Permeabilized SPOC1 cells were exposed to PMA or 4alpha-phorbol at Ca2+ activities ranging from 10 nM to 10 microM. PMA, but not 4alpha-phorbol, increased mucin release at all Ca2+ activities tested: at 10 nM Ca2+ mucin release was 2.1-fold greater than control and at 4.7 microM Ca2+ mucin release was maximal (3.6-fold increase). PMA stimulated 27% more mucin release at 4.7 microM than at 10 nM Ca2+. Hence, SPOC1 cells possess Ca2+-insensitive, PKC-dependent, and Ca2+-dependent PKC-potentiated pathways for mucin granule exocytosis.

Effects of adenosine 3':5'-cyclic monophosphate and guanine nucleotides on calcium-evoked ACTH release from electrically permeabilized AtT-20 cells

1. The mouse AtT-20/D16-16 anterior pituitary tumour cell line was used as a model system for the investigation of adenosine 3':5'-cyclic monophosphate (cyclic AMP)-mediated enhancement of calcium-evoked adrenocorticotrophin (ACTH) secretion. 2. AtT-20 cells were permeabilized by subjecting the cells to intense electric fields. Exposure of permeabilized cells to calcium (1 mM) in the external medium significantly stimulated ACTH secretion over the first 20 min of exposure. This calcium-stimulated ACTH secretion was dependent upon the presence of MgATP (5 mM). 3. The amount of ACTH secreted, in a 20 min incubation at 37 degrees C, from permeabilized cells depended upon the free calcium concentration in the permeabilization medium. Calcium stimulated ACTH secretion from permeabilized cells in the concentration range of 10(-7)-10(-5) M (half maximal = 7 x 10(-7) M). Cyclic AMP(10(-4) M) increased the amount of ACTH secreted at each effective concentration of calcium. However, cyclic AMP did not alter the potency of calcium as a stimulant of ACTH secretion. 4. Guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S, 10(-4) M) stimulated ACTH secretion from permeabilized cells in the absence of calcium and was additive with calcium-evoked ACTH secretion up to a maximum which could be stimulated by calcium acting singly. Guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S, 10(-4) M) inhibited calcium-evoked ACTH secretion from permeabilized cells. 5. GTP-gamma-S stimulated ACTH secretion from permeabilized cells in a concentration-dependent manner. The nucleotide significantly stimulated ACTH secretion at concentrations of 10-'M and above. Cyclic AMP (10-4 M) increased the amount of ACTH secretion evoked by effective concentrations of GTP-y-S.6. The results of the present study support the hypothesis that, in AtT-20 cells, cyclic AMP is acting at some site, distal to calcium entry, which modulates the ability of an increase in cytosolic calcium concentration to stimulate ACTH secretion. One such site may be a GTP-binding protein which the present study suggests may mediate the effects of calcium upon the secretory apparatus. These GTP-binding proteins may be a target for regulation by cyclic AMP.

ATP-dependent and ATP-independent pathways of exocytosis revealed by interchanging glutamate and chloride as the major anion in permeabilized mast cells

Most investigations of the mechanism of regulated exocytosis have involved the use of secretory cells permeabilized in glutamate-based electrolyte solutions. In our previous work we have used NaCl-based electrolyte solutions. For secretion to occur from rat mast cells under these latter conditions, a dual effector system comprising Ca2+ and a guanine nucleotide are required; together they are sufficient. Here we compare the secretion from mast cells permeabilized in solutions of different electrolytes. Replacement of Na+ by K+ had little effect. Replacement of Cl- by Br-, SO4-, gluconate, isethionate, acetate, tartrate, succinate, etc. affected the maximal extent of secretion elicited by the dual effectors Ca2+ and guanosine-5'-O-(3-thiotriphosphate) (Ca2(+)-plus-GTP-gamma-S) but had little influence on the effective affinity for Ca2+. The dicarboxylic amino acids (L- and D-glutamate, and L-aspartate) permitted exocytosis to be elicited by Ca2+ or GTP-gamma-S alone. Secretion stimulated by GTP-gamma-S is strongly inhibited by Cl- (50% inhibition by 20 mM Cl-), whereas the extent of Ca2(+)-induced secretion is proportional to the concentration of glutamate in mixed electrolyte buffers. Unlike dual-effector stimulation, secretion due to the single effectors requires adenosine triphosphate (ATP) and is prevented by inhibitors of protein kinase C. These results point to the existence of two parallel pathways for control of exocytosis in permeabilized cells, one ATP dependent, the other ATP independent.

Adenosine 3',5'-cyclic monophosphate-mediated enhancement of calcium-evoked prolactin release from electrically permeabilised 7315c tumour cells

1. The 7315c tumour cell was used as a model system for the investigation of adenosine 3',5'-cyclic monophosphate (cyclic AMP)-mediated enhancement of calcium-evoked prolactin release. 2. 7315c cells were permeabilised by subjecting the cells to intense electric fields. Studies investigating the penetration of the dye ethidium bromide indicated that the cells were completely permeabilised after 2 discharges of 2000 volts and that the pores remained open for at least 30 min before beginning to reseal. These permeabilisation parameters were consistent with those which gave maximal calcium-stimulated prolactin release. 3. In the absence of calcium and in the presence of EGTA (1 mM), permeabilised 7315c cells secreted prolactin at a rate of 0.23 ng min-1 per 10(6) cells. When EGTA was replaced by 1.5 mM calcium, permeabilised cells secreted prolactin at a rate of 2.20 +/- 0.30 ng min-1 per 10(6) cells in the first 5 min of exposure. Maximal calcium-dependent prolactin secretion from permeabilised cells occurred at 37 degrees C. 4. The amount of prolactin secreted, in a 5 min incubation at 37 degrees C, from permeabilised cells depended upon the free calcium concentration in the permeabilisation medium. Calcium stimulated prolactin release from permeabilised cells in the concentration range 0.1-10 microM (half maximal = 5.8 microM). When permeabilised cells were exposed to cyclic AMP (100 microM) for 5 min prior to and during a 5 min challenge with various concentrations of calcium, the amount of prolactin secreted at each effective concentration of calcium was increased. However, cyclic AMP did not alter the potency of calcium as a stimulant of prolactin secretion. 5. The results suggest that cyclic AMP potentiates calcium-evoked secretion from 7315c cells, not by increasing the entry of calcium into the cytosol, but at a step in the secretory process, distal to calcium entry, which modulates the ability of an increase in cytosolic calcium concentration to stimulate prolactin release.

Dual regulation of ACTH secretion by guanine nucleotides in permeabilized AtT-20 cells

1. We have examined the effects of guanine nucleotides on ACTH secretion from digitonin-permeabilized AtT-20 cells, with the aim of analyzing the involvement of GTP-binding proteins (G proteins) in the secretory process. 2. AtT-20 cells permeabilized with 20 microM digitonin displayed calcium-dependent secretion. The EC50 of calcium was approximately 2 microM and the maximal stimulation was 350% of basal release. 3. Nonhydrolyzable guanine nucleotides also stimulated ACTH release, in a virtually Ca2+-free medium. The EC50 of guanosine 5'-(3-O-thio)triphosphate (GTP gamma S) was approximately 15 microM and the maximal stimulation was approximately 230% of basal release. The effects of calcium and guanine nucleotides were not additive. 4. In the presence of the inhibitory hormone, somatostatin guanine nucleotides inhibited the calcium-stimulated secretion. 5. Both the stimulatory and the inhibitory effects on secretion of guanine nucleotides were independent of changes in cyclic AMP (cAMP) and calcium. It is suggested that G proteins influence an unknown step in the secretion process, which would be near or at the exocytotic site. 6. The results can be explained by assuming the existence of two types of G proteins, one with stimulatory effects on exocytotic release (GeS) and another with inhibitory effects (GeI).

Dopamine receptors: molecular structure and function

Two distinct categories of dopamine receptors, termed D1 and D2, have been identified on the basis of pharmacological and biochemical criteria. Some of the progress made in our understanding of the subunit structure, function and signal transduction properties of these important membrane proteins are reviewed.