STIMULUS-SECRETION COUPLING IN A NEUROSECRETORY ORGAN: THE ROLE OF CALCIUM IN THE RELEASE OF VASOPRESSIN FROM THE NEUROHYPOPHYSIS

In vitro dopamine biosynthesis in the median eminence of rat hypothalamic slices: possible involvement of a Na+-Ca2+ exchange mechanism

Equimolar replacement of Na+ in medium with choline chloride or sucrose and experimental manipulations known to increase [Na+]i, such as ouabain addition and K+ deprivation from medium, caused a marked increase in in vitro DOPA synthesis in the median eminence of rat hypothalamic slices in a Ca2+-dependent manner. These results suggest that a Na+-Ca2+ exchange mechanism is closely involved in the regulation of dopamine biosynthesis in tuberoinfundibular neurons.

Calcium-dependent action potentials in rat supraoptic neurosecretory neurones recorded in vitro

Intracellular recordings obtained from thirty-nine supraoptic nucleus neurones in perfused hypothalamic explants displayed a mean resting membrane potential of -69 mV and spike amplitude of 79 mV. Voltage-current plots were linear in the hyperpolarizing direction and revealed a mean slope resistance of 197 M omega. After Na+ channel blockade with tetrodotoxin (TTX; 0.3-16 microM), the voltage-current relationship did not change significantly for hyperpolarizing pulses. An increase in spike threshold in TTX permitted visualization of a reduction in slope resistance to depolarizing current pulses. This rectification was reduced by the addition of the Ca2+ channel blockers Cd2+, Co2+ or Mn2+. High threshold TTX-resistant spikes with amplitudes ranging between 25 and 64 mV were evoked in an all-or-none manner by brief intracellular current pulses applied above 1.0 Hz. Current pulses presented at lower frequencies (less than or equal to 0.5 Hz) evoked graded responses. In seventeen of nineteen cells, prolonged depolarizing current pulses elicited repetitive firing of TTX-resistant spikes with a progressive increase in their amplitude, rise and fall times and after-hyperpolarizations. TTX-resistant spikes were reversibly abolished when CaCl2 was replaced by equimolar amounts of EGTA or by the addition of either Cd2+, Co2+ or Mn2+ to the perfusion medium. In artificial medium containing EGTA, both the shoulder on the repolarization phase of the spike and the subsequent after-hyperpolarization were reversibly abolished. Tetraethylammonium (TEA; 2-5 mM) induced prolongation of mean action potential durations from 1.9 to 12.3 ms (nineteen cells); in TTX, TEA also prolonged the duration and increased the over-all peak amplitude of the TTX-resistant (Ca2+) spike. While TEA also enhanced the amplitude of the Na+ spike, no comparable prolongation in spike duration was observed. These data indicate that somatic action potentials of supraoptic nucleus cells arise from the co-activation of a low threshold Na+ conductance and a higher threshold Ca2+ conductance; the latter is expressed as a shoulder on the repolarization phase of the action potential.

Forskolin: its effects on potassium-evoked release of vasopressin from the rat neurohypophysis

The effect of forskolin, added either before or 5 min after the onset of potassium-evoked release of vasopressin from isolated neurointermediate lobes of the rat has been investigated. A low concentration of forskolin (1 microM), added before stimulation, enhanced the potassium-evoked release of vasopressin throughout two successive 5 min periods of stimulation. Higher concentrations of forskolin (10-80 microM) produced no effect on the potassium-evoked release of hormone during the first 5 min of stimulation, but inhibited release during the second 5 min period. When added 5 min after the onset of potassium stimulation, forskolin (1-80 microM) reduced the amount of vasopressin released during the remaining 5 min of stimulation. Forskolin produced a concentration-dependent increase in cyclic AMP during both the control and potassium stimulation periods. The amount of cyclic AMP generated by forskolin during potassium stimulation was less than that produced during the corresponding control periods.

Forskolin inhibits potassium-evoked release of vasopressin from rat neurohypophyses

The effects of forskolin, an activator of adenylate cyclase, on the release of vasopressin from isolated rat neurointermediate lobes during a 10 min period of potassium stimulation were investigated. Forskolin was added 5 min after the onset of potassium stimulation. A concentration-dependent reduction in the amount of hormone released during the remaining 5 min period of stimulation was observed. The results suggest that an increase in cyclic AMP following depolarisation inhibits hormone release. This contrasts with several other reports showing that an increase in cyclic AMP prior to stimulation enhances release. We therefore propose that cyclic AMP plays a dual role in stimulus-secretion coupling in the neurohypophysis.

Effects of nicardipine on aldosterone release and pressor mechanisms

This study evaluated the effects of nicardipine, following intravenous infusion and oral administration, on the pressor and aldosterone responses to infused angiotensin II. Six healthy, normotensive male subjects were studied. Following administration of nicardipine, no significant change in blood pressure was seen. Nicardipine attenuated the pressor response produced by intravenous administration of angiotensin. Nicardipine did not inhibit the aldosterone response to angiotensin.

The modulation of placental lactogen secretion by calcium: studies with cultured human term trophoblast

Previous reports have documented the negative regulatory effect of calcium ion on the secretion of human placental lactogen by the placenta. Human term trophoblasts were dispersed from term placenta and maintained in monolayer cell culture. Incubation of the cultured trophoblast with EGTA produced a dose-dependent stimulation of placental lactogen secretion. The calcium ionophore A23187 inhibited hormone secretion. This inhibitory effect was eliminated by the addition of the calcium-calmodulin complex inhibitor trifluoperazine. It is concluded that calcium exerts a negative regulatory effect on the secretion of placental lactogen by a mechanism which requires the action of the calcium-calmodulin complex.

Ultracytochemical localization of Ca++-ATPase activity in pituicytes of the neurohypophysis of the guinea pig

Ca++-ATPase activity (cf. Ando et al. 1981) was examined both light- and electron-microscopically in the neurohypophysis of the guinea pig. Apart from a strong activity within the walls of the blood vessels, in the parenchyma of the neurohypophysis the reaction product of the Ca++-ATPase activity was restricted to the plasmalemma of the pituicytes. This reaction was completely dependent upon Ca++ and the substrate, ATP; the reaction was inhibited by 0.1 mM quercetin, an inhibitor of Ca++-ATPase. A reduction of the enzyme activity occurred by 1) adding Mg++ to the standard incubation medium, and 2) substituting Ca++ with Mg++ at varying concentrations. In all experiments the neurosecretory fibers were devoid of Ca++-ATPase activity. The function of the Ca++-ATPase activity in the plasmalemma of the pituicytes is discussed in connection with the regulation of the extracellular Ca++ concentration, which seems to be important with respect to the discharge of secretory material from the neurosecretory fibers.

Effects of veratridine, tetrodotoxin and other drugs that alter electrical behaviour on secretion of melanocyte-stimulating hormone from melanotrophs of the pituitary pars intermedia

Since melanotrophs are electrically active and exhibit spontaneous Na spikes, a study was made of the effects, on melanotroph secretion, of drugs known to influence electrical properties. The output of melanocyte-stimulating hormone was measured from perifused neurointermediate lobes of mice or melanotrophs dispersed from such lobes of mice or rats. Veratridine (200 microM), which is known to increase Na permeability in a variety of cells, caused a large, although transient, increase in secretion from the melanotrophs that required extracellular Ca2+ and was blocked by the Na-channel blocker tetrodotoxin (1 microM). Tetraethylammonium (10 mM), which blocks K channels and thus prolongs the duration of the action potential in many cells, also stimulated secretion in the melanotrophs in a Ca-dependent manner. This response was not, however, blocked by tetrodotoxin, and is thus not attributable to prolongation of Na spikes in these cells. Moreover, tetrodotoxin did not inhibit basal secretion. The stimulant effect of veratridine on secretion in melanotrophs and its suppression by tetrodotoxin suggests that voltage-dependent Na channels can participate in the regulation of hormone output in these cells of the pituitary pars intermedia. However, the apparent lack of effect of tetrodotoxin on basal secretion suggests that the spontaneous Na spikes previously observed in these cells are not required for promoting the Ca influx which other evidence shows is important for basal secretion.

K+-induced thyrotropin-releasing hormone release from superfused mediobasal hypothalami in rats. Inhibition by somatostatin

Somatostatin (SRIF), in concentration of 10(-6) M, significantly inhibited the depolarization-induced release of immunoreactive thyrotropin-releasing hormone (IR-TRH) from superfused mediobasal hypothalami (MBH) containing mainly the median eminence (ME), without affecting the basal release of TRH. The total amount of K+-induced TRH release was 0.24 +/- 0.02 and 0.61 +/- 0.08 pg/MBH/min, respectively, in the presence and absence of SRIF in the medium. The data are consistent with a role of SRIF as a neuromodulator on TRH release from the ME. In contrast, superfusion with Locke medium containing triiodothyronine (10(-6) M) had no effect on basal and K+-induced IR-TRH release in our system.

Phosphodiesterase activities for cyclic nucleotides in nerve endings from the bovine posterior pituitary gland

The cyclic nucleotide phosphodiesterase (PDE) activities were studied in a nerve ending fraction from bovine neural lobes. Most of the activity was particulate and unaffected by calcium. Lineweaver-Burk plots for this fraction showed negative cooperativity with apparent Km values for cyclic AMP of 11 microM and for cyclic GMP of 4 microM. The soluble activities for both cyclic nucleotides were activated by calcium and inhibited by calmodulin-binding drugs (trifluoperazine and calmidazolium). The apparent Km values were 50 microM for cyclic AMP and 20 microM for cyclic GMP for the soluble activities. Sucrose density gradients resolved the soluble activities into two peaks. The activity with the higher sedimentation rate (MW 122,000 daltons) hydrolysed both cyclic nucleotides and was calcium-calmodulin-dependent. The other peak (MW 47,000 daltons) had a higher affinity for cyclic AMP than for cyclic GMP and was calcium-independent. Solubilized particulate activities gave two main peaks on the density gradient, both calcium-independent. One was mainly for cyclic AMP (MW 47,000 daltons) and the other mainly for cyclic GMP (MW 133,000 daltons). The function of PDEs in relation to secretion was discussed.

Optical recording of action potentials from vertebrate nerve terminals using potentiometric probes provides evidence for sodium and calcium components

Optical methods are shown to monitor action potentials from a population of nerve terminals in the neurohypophysis of Xenopus. Calcium antagonists such as cadmium and nickel ions block a component of the action potential that probably reflects a calcium-mediated potassium conductance, and tetrodotoxin blocks an inward sodium current, revealing a calcium component to the action potential upstroke.

Calcium-containing mitochondrial granules in neurohypophysial axon terminals disappear following vasopressin treatment of Brattleboro rats

The presence of intramitochondrial calcium-containing electron-dense granules was demonstrated in axon terminals of chronically hyperactive neurosecretory neurons of untreated homozygous Brattleboro rats. Following vasopressin treatment for 30 days, which has been shown to attenuate this neuronal hyperactivity, calcium-containing deposits could not be detected in mitochondria. It is concluded that the presence of intramitochondrial calcium-containing dense deposits is connected with the functional state of neurosecretory neurons.

Differential inhibitory action by morphine on the release of oxytocin and vasopressin from the isolated neural lobe

Simultaneous release of both oxytocin and vasopressin was evoked by electrically stimulating the isolated neural lobe. Morphine inhibited the electrically evoked release of both oxytocin and vasopressin, but whereas the opiate antagonist naloxone reversed the suppression of oxytocin release it was without effect against the opiate block of vasopressin secretion. Thus, although the secretion of both neurohypophysial hormones can be suppressed by opiates, the mechanisms may involve different types of receptors. We also provide evidence that, at least for the oxytocin neurones, the presence of endogenous opioid peptides in the neural lobe limits the amount of hormone released by electrical stimulation.

Release of immunoreactive TRH in serum-free cultures of mouse hypothalamic cells

Serum-free cultures of mouse hypothalamic cells were used as a model for studying TRH (thyroliberin) secretion in vitro. Supplementation of the culture medium with triiodothyronine, corticosterone and polyunsaturated fatty acids is necessary to obtain a substantial release capacity of TRH neurons. Under these conditions depolarization of the cells with 60 mM K+ results in a calcium-dependent release of immunoreactive TRH.

Plasma and platelet vasopressin in essential hypertension and congestive heart failure

In this study we found that, in 31 normal subjects, close to 90% of circulating arginine vasopressin (AVP), measured by radioimmunoassay, was associated with platelets. By using routine methods of centrifugation, which do not completely separate platelets, the normal range of plasma vasopressin was higher by twofold than the normal range in platelet-free plasma prepared by differential centrifugation, which was 1.4 +/- 1.0 sd pg/ml. Platelet vasopressin was 12.9 +/- 5.7 pg/ml. Patients with congestive heart failure had, on average, an elevated platelet-free plasma AVP, as did two patients with thrombocytopenia and one with thrombocytosis. Patients with essential hypertension had slightly high levels of platelet-free plasma AVP and demonstrated an abnormal inverse relationship between platelet-free plasma AVP and serum osmolality. Immunoreactive platelet vasopressin was slightly low in patients with essential hypertension and was subnormal in patients with congestive heart failure. These studies demonstrate that platelets normally present in centrifuged plasma cause an overestimation of the plasma vasopressin levels. Until the physiological meaning of plasma and platelet-bound AVP is understood, studies of circulating vasopressin should probably assess both plasma and platelet AVP levels.

Evidence for a cyclic GMP mechanism in the mediation of hippocampal post-tetanic potentiation

Correlative electrophysiological and biochemical techniques were used to study hippocampal post-tetanic potentiation in acutely prepared rabbits following stimulation of the medial septal region and contralateral hippocampal field CA3. The results indicate that calcium ions, guanosine-3':5'-monophosphate, and phosphodiesterase inhibitors selectively enhanced the duration of post-tetanic potentiation. Potassium ions selectively enhanced tetanic potentiation. Adenosine-3':5'-cyclic monophosphate suppressed both tetanic and post-tetanic potentiation. The electrophysiological findings were supported by biochemical observations that guanosine-3':5'-monophosphate levels show marked increases following tetanic stimulation of either the medial septal region or contralateral hippocampal field CA3 pathways. The data suggest that a calcium-dependent process in the presence of a guanosine-3':5'-monophosphate mechanism promotes periods of hippocampal pyramidal cell hyperexcitability. The mechanism by which the cyclic nucleotide alters potentiation does not appear to be coupled to a single receptor variety.

Vasopressin and oxytocin release in the brain--a synaptic event

In order to obtain evidence for a central release of vasopressin and oxytocin, the release of these peptides was demonstrated in various extrahypothalamic areas of the rat brain. It proved that in those areas where these peptidergic fibers terminate synaptically a vasopressin and/or oxytocin calcium-dependent release, similar to that in the neurohypophysis, could be evoked by potassium or veratridine. Such release was not found in areas in which these fibers do not exhibit synaptic specialization.

Regulation of phosphorylation of proteins I, IIIa, and IIIb in rat neurohypophysis in vitro by electrical stimulation and by neuroactive agents

The state of phosphorylation of proteins I, IIIa, and IIIb--neuron-specific phosphoproteins--was studied in neurosecretory endings of the neurohypophysis in vitro. Brief periods (a few seconds) of electrical stimulation caused large increases in the state of phosphorylation of all three proteins. The three proteins were dephosphorylated within 1 min after termination of the stimulation. High potassium, 8-bromo-cAMP, and dopamine also stimulated the phosphorylation of the three proteins. The effect of dopamine was blocked by the dopamine antagonist fluphenazine. Peptide mapping of protein I revealed that electrical stimulation or high potassium increased the state of phosphorylation of two regions of the molecule, whereas 8-bromo-cAMP and dopamine increased the state of phosphorylation of only one of these regions.

Cyclic nucleotides and the release of vasopressin from the rat posterior pituitary gland

When rat posterior pituitary glands were stimulated by a high concentration of potassium, a peak of cyclic AMP and a peak of cyclic GMP were detected after 0.5 min and 1 min, respectively, whereas the rate of release of vasopressin was maximal only after 2 min. When calcium was omitted from the medium, no significant changes in cyclic nucleotide levels were found and the vasopressin release remained at the basal rate. During cold-stimulated (10 degrees C) release of vasopressin, a peak of cyclic AMP was detected after 5 min simultaneously with the maximal rate of vasopressin release. The significance of the cyclic nucleotides in the release of vasopressin is discussed.

Morphological evidence for the involvement of calcium in neurohypophysial hormone release

An elevated intracellular Ca2+ concentration in the secretory nerve endings of the rat neurohypophysis was detected histochemically by means of light microscopy concomitant with the vasopressin secretion evoked by hypertonic saline. The electron microscopic and X-ray microanalytical results furnish morphological evidence for the function-dependent Ca2+ storage capacity of the mitochondria, and suggest their role in the regulation of the free Ca2+ level in the neurosecretory axon terminal.

Acetylcholine, melatonin, and potassium depolarization stimulate release of luteinizing hormone-releasing hormone from rat hypothalamus in vitro

We have examined the release of radioimmunoassayable luteinizing hormone-releasing hormone (LH-RH) from fragments of rat medial basal hypothalamus. These fragments were cultured overnight in medium containing serum and then preincubated in groups of three for 10 min in medium resembling cerebrospinal fluid in its electrolyte constituents and containing bacitracin. This was followed by 30-min incubation periods during which some of the hypothalami were exposed to test substances. Potassium depolarization, effected by the addition of 56 mM potassium chloride to the incubation medium, caused a marked stimulation in LH-RH release, but only in the presence of calcium. Acetylcholine at 10 nM and the parasympathomimetic anticholinesterase agent neostigmine at 1 microM markedly stimulated LH-RH release. Hexamethonium, a nicotinic antagonist, at 1 microM abolished the acetylcholine-induced increment in LH-RH release. Melatonin, a pineal indolamine, caused significant stimulation of LH-RH release at a concentration as low as 10 nM. Bacitracin (21 microM) was employed in all these experiments. It had no effect on LH-RH release but did prevent the degradation of LH-RH in this system. We conclude that acetylcholine and melatonin are capable of inducing LH-RH release from the rat medial basal hypothalamus. These actions may account for some of the progonadotropic properties previously ascribed to these agents.

The molluscan neuropeptide FMRFamide: calcium-dependent release and blood levels in Macrocallista (Bivalvia)

Ganglia dissected from the marine bivalve, Macrocallista nimbosa, were pooled and incubated in a series of seawater media designed to promote or inhibit the release of the neuropeptide Phe-Met-Arg-Phe-NH2 (FMRFamide). The amount of peptide released, determined by radioimmunoassay (RIA), was enhanced two to seven times when the medium contained high potassium. FMRFamide released was reversibly inhibited when calcium was omitted from the high potassium medium. Immunoreactive FMRFamide was found in Macrocallista hemolymph in a concentration (10(-9)M) that excites isolated Macrocallista ventricles. Although the in vivo release sites have not yet been localized, these findings already show that the endogenous neuropeptide FMRFamide can be characterized as a neurohormone or as a neurotransmitter in Macrocallista.

The relationship between the membrane potential of neurosecretory nerve endings, as measured by a voltage-sensitive dye, and the release of neurohypophysial hormones

The membrane potential of isolated rat neurohypophyses and isolated neurosecretosomes (neurosecretory nerve endings) was monitored with the voltage sensitive fluorescent probe diS-C3-(5). K ions, in contrast to Na or Cl ions, give rise to large changes of the fluorescent signal. The fluorescent response is linearly related to log[K+]0 at values higher than 10 mM, whereas at lower [K+]0 the permeability of the membrane for Na ions has to be taken into account. Veratridine increases the fluorescent signal only in the presence of external sodium; this effect is blocked by tetrodotoxin. After prolonged K-induced depolarisation, addition of veratridine to the medium gives a further change in fluorescence of diS-C3-(5) associated with release of vasopressin. Vasopressin release from isolated neurohypophyses started to increase significantly only above 25 mk [K+]0, while the depolarization of the membrane was linearly related to log[K+]0. The results are consistent with the view that neurosecretory nerve endings have voltage-dependent calcium channels that regulate the amount of hormone released during depolarisation.

Lack of effect of opiates on release of vasopressin from isolated rat neurohypophysis

Electrical stimulation of rat neurohypophysis in vitro results in a 150-fold increase in release of vasopressin. Incubation of the isolated lobes with endogenous opiates (Met-enkephalin, Leu-enkephalin, beta-endorphin), morphine or naloxone did not change the evoked release of vasopressin significantly. The results obtained do not confirm the hypothesis that endogenous opiates inhibit the release of vasopressin from this preparation.

Calcium efflux from the rat neurohypophysis

1. Calcium efflux from isolated rat neurophypophyses has been studied. Curve fitting of the wash-out curves suggests three phases with t((1/2)) of ca. 3, 15 and 130 min.2. The slow component of the (45)Ca efflux is attributed to efflux of intracellular Ca. On the basis of the temperature sensitivity of the Ca efflux, the activation energy has been calculated to be approximately 12,000 cal/mole, corresponding to a Q(10) of ca. 2.0.3. Ca efflux decreased by approximately 32% when external Na was replaced by choline. Li(o), in the presence or absence of Ca(o), was as effective as Na(o) in stimulating the Ca efflux.4. The curve relating Ca efflux to [Na](o) or [Li](o) is sigmoid and suggests that at least two Na (or Li) ions are necessary to activate the efflux of each Ca ion. Ca(o) does not modify the absolute Na-dependent Ca efflux but decreases the affinity for Na of the site involved in Ca extrusion.5. Removal of Ca(o) decreased the Ca efflux by ca. 44% in Na-free media. The apparent affinity for Ca(o) of the Ca(o)-activated Ca efflux (K(m) (Cao) = 20 muM) is greatly decreased by the presence of 150 mM-Na (K(m) (Cao) = 0.8 mM).6. Lanthanum decreased the total Ca efflux by ca. 60% and totally abolished the Na(o)-activated and Ca(o)-activated Ca efflux.7. Vanadate reduced the Ca efflux remaining in Na-, Ca-free saline by 73%.8. Elevation of Na(i) with ouabain did not modify the rate of loss of (45)Ca.9. Increased concentration of K(o) stimulated transiently the (45)Ca loss. The time course of this increase depends on the Ca(o) concentration ([Ca](o)).10. Cyanide or CCCP (carbonyl cyanide m-chlorophenylhydrazone) increased transiently the Ca efflux. The increase induced by cyanide could only be observed when the neural lobes had been over-loaded with (45)Ca.11. Membrane destruction induced by high temperature eliminated the effect of [Na](o) and [Ca](o) on (45)Ca efflux.12. In 150 mM-Na-containing saline, half-maximum activation of (45)Ca uptake occurs in the 0.2-0.4 mM [Ca](o) range.13. The Ca efflux from isolated pituicytes was not affected by removal of Na(o).14. In conclusion we show that Ca efflux from neurosecretory nerve terminals can be subdivided into three components of approximately the same magnitude, one which is activated by Na(o), another by Ca(o) and a third component which is independent of Na(o) and Ca(o).

Neurotensin stimulates exocytotic histamine secretion from rat mast cells and elevates plasma histamine levels

1. Neurotensin stimulated histamine release and granule extrusion when applied to isolated rat peritoneal mast cells. 2. This secretory response was prevented by the removal of calcium or energy and was not accompanied by the release of lactic dehydrogenase. 3. The secretory response produced by neurotensin was prevented by prior treatment of mast cells with cromoglycate. 4. The intravenous injection of neurotensin into anaesthetized rats produced a rapid and significant increase in the level of blood histamine that was dependent upon the dose of neurotensin. 5. Treatment of rats with compound 48/80, 24 hr before neurotensin, abolished the elevation in blood histamine caused by neurotensin. The intravenous injection of cromoglycate 1-2 min before neurotensin greatly reduced the response to neurotensin. 6. The intradermal injection of neurotensin (0.03-30 p-mole) increased capillary permeability in rats pre-treated intravenously with Evans Blue. This response was abolished by the antihistamine, diphenhydramine. Increasing the dose of neurotensin to 300 p-mole partially overcame this inhibition by diphenhydramine. 7. Our results demonstrate that neurotensin can elicit an exocytotic secretory response from isolated rat peritoneal mast cells and elevate histamine levels in blood. It is suggested that some of neurotensin's physiological effects may be due to stimulation of mast cell secretion.

Calcium dependency of potassium-stimulated thyrotropin-releasing hormone secretion from rat neurohypophysis in vitro

The role of Ca2+ on the K+ -stimulated release of thyrotropin-releasing hormone (TRH) from rat neurohypophysis, stalk median eminence (SME) and hypothalamic fragments in vitro was studied. High K+ (50 mM) caused a significant increase (2-6-fold) in TRH release from all three tissues, (P less than 0.05-0.01), and this effect was inhibited (by 62-80%) on removing Ca2+ (1.8 mM) from the medium (P less than 0.01). The calcium dependency of the TRH response to a membrane depolarizing stimulus (high K+) is consistent with TRH release occurring by a neurosecretory process from the neurophypophysis, as well as from hypothalamic tissue.

Exocytosis of pinocytosed fluid in cultured cells: kinetic evidence for rapid turnover and compartmentation

The uptake and fate of pinocytosed fluid were investigated in monolayers of pulmonary alveolar macrophages and fetal lung fibroblasts using the fluid-phase marker, [14C]sucrose. Initial experiments revealed that cellular accumulation of chromatographically repurified [14C]sucrose was not linear with incubation time. Deviation from linearity was shown to be due to constant exocytosis of accumulating marker. Chromatographic analysis revealed that the cells were unable to metabolize sucrose and were releasing it intact by a process that was temperature-sensitive but not dependent on extracellular calcium and magnesium. A detailed analysis of the kinetics of exocytosis was undertaken by preloading cells with [14C]sucrose for various lengths of time and then monitoring the appearance of radioactivity into isotope-free medium. Results indicated that modeling the process of fluid-phase pinocytosis and subsequent exocytosis required at least two intracellular compartments in series, one compartment being of small size and turning over very rapidly (t1/2 = 5 min in macrophages, 6--8 min in fibroblasts) and the other compartment being apparently larger in size and turning over very slowly (t1/2 = 180 min in macrophages, 430--620 min in fibroblasts). Computer-simulation based on this model confirmed that the kinetics of efflux faithfully reflected the kinetics of influx and that the rate of efflux completely accounted for the deviation from linearity of accumulation kinetics. Moreover, the sizes of the compartments and magnitude of the intercompartment fluxes were such that the majority of fluid internalized in pinocytic vesicles was rapidly returned to the extracellular space via exocytosis. This result provides direct experimental evidence for a process previously thought necessary based solely on morphological and theoretical considerations. Furthermore, the turnover of pinocytosed fluid was so dynamic that accumulation deviated from linearity even within the first few minutes of incubation. We were able to show that the kinetics of exocytosis allowed calculation of the actual pinocytic rate, a rate that was nearly 50% greater than the apparent initial rate obtained from the slope of the uptake curve over the first 10 min.

Changes in posterior pituitary oxytocin release in vitro during the estrous cycle of female rats

The in vitro release of oxytocin (OXT) from posterior pituitary lobes (PPL) of adult female rats is linked to the stage of the animals' sexual cycles. After incubation in normal Locke's solution (K+ 5,6 mM), the basal OXT release from the PPL of rats on diestrus 1 and estrus amounted to 2.25 +/- 0.53 ng/lobe/10 min (mean +/- S.E.M.) and 4.71 +/- 0.61 ng/lobe/10 min, respectively. Excess K+ (56 mM/l) in the presence of Ca++ (2.2 mM/l) increased OXT liberation from the PPl of diestrous and estrous rats to 12.41 +/- 2.65 ng/lobe/10 min und 36.33 +/- 6.18 ng/lobe/10 min, respectively. When Ca++ was omitted from the incubation medium, the K+-excess induced OXT release from the PPL of estrous rats decreased to 21.54 +/- 2.65 ng/lobe/10 min whereas no change occurred in the OXT-release from the rats' PPL on diestrus 1 in the presence of Ca++. The results indicate a cycle-dependent release and suggest that OXT plays a role in the reproductive processes.

Oxytocin release from isolated posterior pituitary lobes of adult male rats as determined by radioimmunoassay

In vitro oxytocin (OXT) release from isolated posterior pituitary lobes (PPL) of adult male Wistar rats was measured under basal and K+- stimulated conditions using a specific, sensitive radioimmunoassay. A basal release of 0.95 +/- 0.20 ng OXT/lobe/10 min was estimated in standard Locke's bathing solution. An excess of K+ (56 mmol/l) augmented the OXT secretion to 18.1 +/- 2.24 ng OXT/lobe/10 min in the presence of 2.2 mmol/Ca++. A stimulatory effect of K+ excess was also determined in Ca++-free medium and in Ca++ free medium enriched with 0.7 mmol/l EGTA. An inhibitory effect on K+-stimulated OXT release was achieved by raising the Mg++ concentration from 1.0-8.0 mmol/l of bathing fluid. During prolonged K+ stimulation the rate of OXT release declined exponentially. Estimation of the OXT content of PPLs after prolonged stimulation with K+ excess revealed that the lobes still contained 80% of their original OXT content.

Human placental lactogen release in vitro: paradoxical effects of calcium

To study the effects of calcium on the release of human placental lactogen (hPL), placental explants were exposed to media containing lower or higher concentrations of calcium than normally available to the placenta. Explants exposed for 2 h to calcium-poor medium or medium containing either 2 mM EDTA or 2 mM EGTA released 160, 248, and 253% more hPL, respectively, than control explants. In contrast, explants exposed to medium containing higher than normal calcium concentrations released the same amounts of hPL as the control explants. At lower than normal extracellular calcium concentrations, the increased hPL release was inversely proportional to the calcium concentration. The increased release in calcium-poor medium was inhibited by subsequent exposure of the explants to medium containing calcium and was prevented by either barium or magnesium. Changes in barium or magnesium concentrations, however, had no effects on hPL release in the presence of normal extracellular calcium concentrations. Methoxyverapamil (D 600), an inhibitor of calcium flux, stimulated hPL release. Because low extracellular calcium and methoxyverapamil both inhibit calcium influx, these experiments suggest that calcium influx inhibits hPL release. The role of calcium in the regulation of hPL release therefore appears to be different from that reported in other release systems.

Neurotensin release by rat hypothalamic fragments in vitro

The release of neurotensin by hypothalami from male rats was investigated in vitro using tissue fragments incubated in Krebs-Ringer bicarbonate-glucose buffer at 37 degrees C. Neurotensin was measured by radioimmunoassay using an antibody directed toward the C-terminal portion of the peptide. Neurotensin-like immunoreactivity released into the incubation medium eluted from Sephadex G-25 in a position identical to that of the synthetic peptide and serial dilutions of incubation medium were parallel to those of synthetic neurotensin in the radioimmunoassay. Neurotensin-like immunoreactivity released into the incubation medium was degraded during the incubation period by released hypothalamic peptidases. The addition of bacitracin (0.5 mg/ml) to the medium partially prevented this degradation. Neurotensin release was stimulated by dibutyryl cyclic AMP (10(-4) M) and by depolarizing concentrations of potassium. The latter effect was shown to be Ca2+-dependent. Dopamine (10(-4)--10(-6) M) stimulated neurotensin release in a dose-dependent manner and this effect was blocked by the dopamine receptor antagonist, haloperidol. Neurotensin release was not stimulated by either norepinephrine (10(-4) M) or serotonin (10(-4) M). The results indicate that neurotensin is released by the hypothalamus in vitro; its release is stimulated by membrane depolarization in a Ca2+-dependent manner and may involve an adenylate cyclase mechanism; and dopamine appears to serve as a stimulatory neurotransmitter for neurotensin-containing neurons.

Ultrastructural organization of actin filaments in neurosecretory axons of the rat

The ultrastructural organization of actin filaments was studied in the neurohypophysial system of the rat after heavy meromyosin (HMM) labeling. This structural pattern is characterized by (1) a straight arrangement of the filaments parallel to the axonal axis in the proximal nondilated parts of axons, (2) a central location within axonal dilatations, and (3) a higher concentration within axonal endings where the filaments form a complex three-dimensional network. The relationships of the filaments to other axonal structures and organelles was further studied by use of electron microscopic stereoscopy. The actin filaments frequently appear anchored to the axolemma with either polar arrangements of the arrowhead decoration (i) at structurally undifferentiated sites, and (ii) more particularly within perivascular endings, at sites with electron-dense thickenings. In all axonal divisions actin filaments are also found to bind to filamentous material surrounding the microtubules and to various organelles. Within the terminal portions of the axons actin filaments exhibit close relationships to neurosecretory granules and to the numerous smooth microvesicles found in this region. Such preferential relationships are particularly observed both in axon ;terminals and in pituicytes, with coated vesicles frequently binding actin filaments. In water-deprived rats, the concentration of actin filaments is conspicuously increased along the axons and more clearly in the axonal swellings and endings, where they form a more complex and interconnected network. These data are discussed in the light of a possible involvement of contractile proteins in the mechanisms of axonal transport and terminal release of neurosecretory products.

Discharge of gonadotrophin-releasing hormone from the mediobasal part of the hypothalamus: effect of stimulation frequency and gonadal steroids

The release of gonadotrophin-releasing hormone (Gn-RH), in response to electrical stimulation of the mediobasal part of the hypothalamus incubated in vitro, was studied in both male and female rats. In male rats significant release of Gn-RH occurred during the 10-min experimental period only when the incubated tissue was stimulated at frequencies of 10 Hz or greater. There was no release when stimulated at 5 Hz. There was also no release of hormone when the mediobasal hypothalami were incubated in a Ca2+ free medium. The amount of Gn-RH released during a 10-min incubation period increased progressively as the frequency of stimulation was raised from 10--100 Hz. During short (4-min) incubation periods the effectiveness of each stimulus pulse for Gn-RH release also increased with the frequency of stimulation. However, when stimulated for 10 min there was no increase in hormone released per stimulus pulse when frequency of stimulation was raised above 10 Hz. The amount of Gn-RH released in response to stimulation at 50 Hz was greater in male rats than in females. For the females, there was no significant difference between the amounts of Gn-RH released at dioestrus and pro-oestrus. In both male and female rats gonadectomised 4 weeks prior to hypothalamic incubation, the response to electrical stimulation at 50 Hz was reduced when compared with intact controls. Indeed, for the females there was no longer a statistically significant increase in the amount of Gn-RH in the incubation medium after 50 Hz stimulation. Ovariectomised female rats, injected twice daily for 3.5 days with 20 microgram of oestradiol benzoate released Gn-RH in response to 50 Hz stimulation in the same amounts as intact control animals. By contrast, there was no recovery of Gn-RH release to normal levels in castrated male rats similarly treated with 1.25 mg testosterone propionate.

Opiate receptors influence vasopressin release from nerve terminals in rat neurohypophysis

A previous report described the existence of substantial amounts of enkephalin immunoreactivity and the occurrence of nerve terminals containing an enkephalin-like material in the pars nervosa of rat pituitary. It was suggested that an enkephalin innervation of the pars nervosa originating from the magnocellular hypothalamic nuclei might regulate the secretion of neurohypophyseal hormones. The results of the present studies support this hypothesis, as we find that a stable enkephalin analogue (D-Ala 2,D-Leu5-enkephalin) inhibits the calcium-dependent release of vasopressin evoked by electrical stimulation of the rat pituitary stalk in vitro. A similar inhibition of the stimulus-evoked vasopressin release is caused by morphine and beta-endorphin, and the inhibitory effects of the enkephalin analogue can be reversed by naloxone. These findings suggest the possible existence of inhibitory opiate receptors on the terminals of vasopressin fibres in the pars nervosa.

Role of Ca2+ and cyclic nucleotides in the control of prostaglandin E production in the rat anterior pituitary gland

In an attempt to relate changes in the intracellular concentration of prostaglandin E to the secretion process, two agents known to increase cyclic nucleotide concentrations and hormone release were added to dispersed rat anterior pituitary cells. They caused increases in teh intracellular prostaglandin E concentrations. Increasing the K+ concentration in the medium (which stimulates hormone release) caused a rapid rise in prostaglandin E concentrations. The addition of the Ca2'onophore A23187 had a similar effect. The effects of changes in the K+ and Ca2+concentrations and the addition of EDTA were measured on the redistribution of radioactivity in pituitary glands prelabelled with [3H]arachidonic acid. Elevated K+ concentrations stimulated the transfer of label to prostaglandins and free arachidonic acid, suggesting an increased phospholipase A activity. On the other hand, the absence of extracellular CaCl2 and the addition of EDTA had the opposite effect, which could be cancelled by the addition of sufficient amounts. of CaCl2. It is concluded that the addition of agents that increase membrane permeability to bivalent cations probably results in an influx of Ca2+ and this appears to result in increased phospholipase A activity, which in turn leads to an increase in prostaglandin production.