Faculty Bibliography

Vasopressin (VP) elicits almost identical insulin-stimulatory dose responses in isolated mouse islets and hamster beta (HIT) cells. We have further pharmacologically characterized HIT cell VP receptors by comparing the potencies of a series of VP agonists including the novel V1b agonist, desamino(D-3-(3'-pyridyl)-Ala2,Arg8)VP (d(D-3-Pal)VP), in stimulating insulin secretion and inositol phosphate (IP) production. The relative orders of potency of VP analogues were parallel in both respects: desamino-Arg-VP (dAVP) > Arg-vasotocin (AVT) = VP > oxytocin (OXY) > desamino-D-Arg-VP (dDAVP) > d(D-3-Pal)VP. dAVP, the most potent agonist tested, behaved as a V1 but non-V1a agonist. The potency of d(D-3-Pal)VP relative to VP was 1:134 in stimulating insulin secretion and 1:40 with respect to IP production. In HIT cell monolayers, the relative order of affinity of analogues in competition for binding with [3H]AVP was: dAVP > AVT = VP > V1a antagonist > OXY > dDAVP > V2 antagonist = d(D-3-Pal)VP, in parallel with their biological activity. The relative orders of potency and affinity parallel those reported for corticotrophic V1b receptors. Binding studies with hamster liver membranes indicate that the hepatic VP receptor belongs to the V1a class. We conclude that VP activates phospholipase C and interacts with functional VP receptors of the V1 type, which do not belong to the V1a subclass and which are similar to V1b receptors

Arginine vasopressin (VP) activates phospholipase C (PLC) and stimulates insulin secretion and inositol phosphate (insP) production in mouse islets and clonal hamster beta cells (HIT). The insulin response to VP is reportedly dependent on extracellular glucose and there is evidence that glucose also activates PLC. The interactions of VP and glucose have been further examined in HIT cells. Glucose stimulated insulin secretion but not insP production and VP stimulated both insulin secretion and insP production in the absence of extracellular glucose. However, in the presence of glucose the insulin and insP responses to VP were potentiated. The phorbol ester, tetradecanoylphorbol acetate (TPA), which activates protein kinase C (PKC), stimulated basal insulin secretion but inhibited insP production. In the presence of submaximal concentrations of VP, 100 nmol/l TPA inhibited VP-stimulated inositol monophosphate production and there was no additivity of stimulated insulin secretion. With a maximal (10 nmol/l) concentration of VP, TPA caused additive insulin secretion and insP levels were stimulated above baseline. Twenty-four hour preincubation with TPA to down-regulate PKC did not inhibit the insulin response to VP. We conclude that extracellular glucose does not activate PLC and is not required for VP-stimulated insulin secretion although it potentiates VP-stimulated insulin secretion and insP production

One hundred seventy-eight implants from three systems were placed in 89 type II diabetic patients at 13 Department of Veterans Affairs medical centers. Four failures (2.2 percent) were found at uncovering. The failure rate increased to 7.3 percent at the end of 1 year (nine additional failures). Study patients will be monitored for an additional 4 years. Initial results suggest that type II diabetic patients can be considered for dental implant therapy. © 1994 by Williams and Wilkins.

The temporal interactions of rat GH-releasing factor (GRF) and somatostatin (SRIF) on the secretion of GH from perifused rat anterior pituitary cells have been studied. SRIF and GRF were employed at concentrations in a range close to levels reported in the hypophysial circulation of the rat. GH secretion was inhibited by pulses of 1 nmol SRIF/l (6 min) or 0.3 nmol SRIF/l (6 or 20 min). No rebound GH stimulation was observed. Exposure of cells to 6-min pulses of 0.3 nmol rat GRF/l repeated three times resulted in rapid stimulatory responses which became attenuated. Concomitant exposure to 0.3 nmol SRIF/l during a GRF pulse resulted in transient inhibition followed by a delayed and enhanced GH response, measured as the area under the curve but not peak height, whereas continuation of SRIF after a GRF pulse abolished the GH response. Exposure to 0.3 nmol SRIF/l prior to but not during a GRF pulse did not delay or inhibit the GH response. The area under the curve was increased under these conditions because of the lowered baseline in cells perifused with SRIF. SRIF alters somatotroph responsivity to equimolar concentrations of rat GRF and under different temporal conditions and can inhibit, enhance or delay GH secretion

Vasopressin (VP) stimulates insulin secretion and inositol phosphate (InsP) production in clonal hamster beta cells (HIT) via a cyclic AMP-independent V1-receptor-mediated signal-transduction pathway. Somatostatin (SRIF) inhibited VP-stimulated insulin secretion, and the effects of SRIF were abolished by pretreatment with pertussis toxin. The Ca(2+)-channel blockers verapamil and nifedipine also inhibited VP-stimulated insulin secretion during 20 min incubations, but verapamil was ineffective at 2 min, and the effects of SRIF and nifedipine together were not addictive. SRIF failed to inhibit further the attenuated insulin response to VP in Ca(2+)-free medium. VP-stimulated InsP production was also inhibited by SRIF in a pertussis-toxin-sensitive manner. Whereas VP-stimulated insulin secretion was almost completely inhibited by SRIF at an equimolar concentration, VP-stimulated InsP production was much less sensitive to inhibition by SRIF, even at a 100-fold excess concentration. VP increased cytosolic Ca2+ in HIT cells loaded with fura 2, the fluorescent Ca2+ indicator. The increase was biphasic, with an initial rapid spike increase followed by a prolonged second phase. Both SRIF, at a concentration which inhibited VP-stimulated insulin secretion but not InsP production, and verapamil failed to inhibit the rapid spike increase in intracellular Ca2+, but did inhibit the second phase. We conclude that VP induces biphasic changes in cytosolic Ca2+, secondary to mobilization of intracellular Ca2+ and influx of extracellular Ca2+. SRIF inhibits insulin secretion by interrupting influx of extracellular Ca2+, likely by inhibiting Gi-subunit activity. Inhibition of VP-stimulated phosphoinositide hydrolysis, which is also pertussis-toxin-sensitive, may represent an additional mechanism of action of SRIF

Somatostatin (SRIF), a peptide widely distributed in the central nervous system, has been implicated in the genesis of seizure activity in a number of animal models of epilepsy. We examined the effects of the anticonvulsants, phenytoin, carbamazepine and diazepam, on the release of SRIF from dispersed adult rat neuronal cells in short-term culture. Each of these agents caused dose-dependent inhibition of ouabain-stimulated SRIF release in a well-characterized hypothalamic dispersed cell system. We also examined the effects of phenytoin on SRIF release from dispersed rat cortical cells and inhibition of stimulated SRIF secretion was again observed. These findings support the hypothesis that the inhibition of neuronal SRIF release may represent a pharmacological mechanism of action of anticonvulsants

The precise roles of GH-releasing factor (GRF) and somatostatin (SRIF) in the orchestration of pulsatile GH secretion have not yet been fully determined. We examined the interactions of rat GRF and SRIF in the concentration ranges present in rat hypophysial-portal blood, on the secretion of GH from dispersed male rat anterior pituitary cells in monolayer culture. The effects of exposing cells to GRF and/or SRIF (0.01-1.nmol/l) for 1 h were compared with the effects of preincubation of cells with SRIF before experimental incubations. As anticipated, the stimulatory effects of 0.1-1 nmol GRF/1 were abolished by concurrent incubation with SRIF at an equimolar concentration, although SRIF, at these concentrations, did not significantly inhibit basal GH secretion. Conversely, pre-exposure to 0.1 nmol SRIF/1 for 30 or 60 min, resulted in an increase in GH secretion during a subsequent 60-min incubation period, both in the absence or in the presence of GRF (0.01-1 nmol/l). Pretreatment with GRF caused increased responsivity to GRF rather than significant sensitization of the GH response to GRF. These observations demonstrate actions of SRIF, at low and probably physiological concentrations, which are more complex than those of a pure inhibitor of GH secretion. Pre-exposure of the pituitary to SRIF enhances subsequent GH secretion, suggesting that SRIF may play an additional physiological role in amplifying the GRF signal

The recent isolation of vasopressin (VP) from the rat and human pancreas led us to investigate the effects of VP on insulin secretion. In the SV 40-transformed hamster beta cell line (HIT), 0.1-1.0 nM VP caused rapid stimulation of insulin secretion. Slight but significant inhibition of insulin secretion was observed in the presence of 10 pM VP. These effects of VP on insulin secretion were paralleled by dose-dependent changes in inositol phosphate (IP) production, indicating mediation by V1-type VP receptors. VP stimulated IP3 production at 30 sec and production of IP1 by 60 sec. VP (0.1 nM to 1 microM) failed to stimulate the release or cellular content of cAMP, whereas forskolin was an effective stimulus. Forskolin and VP together caused at least additive stimulation of insulin secretion. Taken together, these observations indicate that VP is not acting via V2-mediated pathways. However, VP-induced stimulation of insulin and IP production were only slightly inhibited by a V1a pressor antagonist in 100- or 1,000-fold excess, indicating that VP effects are not mediated by V1a receptors. The V1 receptor involved may represent a V1b or a novel type of VP receptor. These observations suggest a potential physiological role of VP in regulating insulin secretion

A number of in vivo studies suggest that hypothalamic somatostatin (SRIF) tone is stimulated by the beta-adrenergic system. Employing dispersed adult male rat hypothalamic cells, we studied the effects of beta-adrenergic antagonists on the release of hypothalamic SRIF. Propranolol, at concentrations of 1-100 microM, had no detectable effect on basal SRIF release, but caused dose-dependent inhibition of SRIF release stimulated by ouabain. Two other beta-adrenergic antagonists, labetolol and metoprolol, also caused inhibition of ouabain-stimulated SRIF release. The alpha 2-agonist clonidine was without effect on SRIF release under basal or stimulated conditions. GH secretion from monolayers of dispersed rat anterior pituitary cells was also examined. Propranolol (1-100 microM) had no significant effect on basal GH secretion or GH secretion stimulated by rat GRF. In conclusion, 1) beta-adrenergic antagonists caused inhibition of stimulated SRIF release; 2) clonidine had no detectable effect on SRIF release; and 3) propranolol did not affect GH secretion in vitro. These findings support the hypothesis that beta-adrenergic antagonists augment GH responsivity by inhibiting hypothalamic SRIF release