Biomedical subjects
G R Hart
Publications and source records attributed to G R Hart.
Effects of a novel hypothalamic peptide, pituitary adenylate cyclase-activating polypeptide, on pituitary hormone release in rats.
We have demonstrated that the novel hypothalamic peptide pituitary adenylate cyclase-activating polypeptide (PACAP-38; 0.1-100 nmol/l) caused an increase in the release of GH, ACTH, LH and alpha-subunit and accumulation of intracellular cyclic AMP from dispersed rat anterior pituitary cells in static culture for 24 h. There were no significant effects on TSH or prolactin release over the same time-period. PACAP-38 (10 nmol/l) increased the release of GH by 1.3-fold (P less than 0.05), ACTH by 1.9-fold (P less than 0.05), LH by 3.5-fold (P less than 0.001) and alpha-subunit by 2.0-fold (P less than 0.005) and the accumulation of intracellular cyclic AMP by greater than 2-fold (P less than 0.001) after 24 h. However, the time-course for the effect of PACAP-38 (1 mmol/l) on hormone release and intracellular cyclic AMP levels showed a temporal dissociation. The effect of PACAP-38 on GH and ACTH levels did not reach significance until 24 h whereas the effect of PACAP-38 on LH and alpha-subunit release reached significance after 4 h implying a different mechanism of action for their release. To investigate the PACAP-induced secretion of LH and alpha-subunit further, we examined the effects of PACAP after down-regulation of protein kinase C (PKC). PACAP-38 at a dose maximal for the stimulation of LH and alpha-subunit release (10 nmol/l) added together with the PKC activator, 12-O-tetradecanoyl-phorbol-13-acetate (TPA; 0.1 mumol/l) had no greater effect on LH and alpha-subunit release than TPA alone over a 4 h incubation period.(ABSTRACT TRUNCATED AT 250 WORDS)
Midazolam modifies pancreatic and anterior pituitary hormone secretion during upper abdominal surgery.
We have investigated the effects of midazolam on circulating concentrations of pituitary and pancreatic hormones in eight healthy patients during cholecystectomy and compared the changes with those in a control group of eight patients. Anaesthesia was induced either with midazolam 0.42 mg kg-1 i.v. followed by an infusion of midazolam 0.125 mg kg-1 for 1 h, or with a sleep dose of thiopentone. All patients received vecuronium, and the lungs were ventilated with 0.5-1.0% halothane and 70% nitrous oxide in oxygen. The use of midazolam was associated with a significant decrease (P less than 0.05) in secretion of cortisol and insulin during surgery. Serum growth hormone secretion was augmented, but this did not reach statistical significance. There was little difference in the concentrations of circulating metabolites between the two groups, except for a significant decrease in blood glucose (P less than 0.05) in the midazolam group after 60 min. There were no marked differences in cardiovascular variables between the two groups of patients.
Local analgesia prevents the cortisol and glycaemic responses to cataract surgery.
We studied the metabolic and hormonal responses to cataract surgery in 18 elderly patients, allocated randomly to receive either general anaesthesia or local analgesia by means of a retrobulbar block. Local analgesia prevented the increases in circulating cortisol and glucose concentrations found in those patients who received general anaesthesia. The results show that complete afferent sensory block of the operative site inhibited endocrine and metabolic responses to ophthalmic surgery.
Hormonal responses to cardiac surgery: effects of sufentanil, somatostatin and ganglion block.
We have investigated the effect of the addition of somatostatin and trimetaphan to sufentanil 20 micrograms kg-1 on the hormonal responses to cardiac surgery and compared the changes with a control group receiving sufentanil and sodium nitroprusside. Eighteen patients undergoing elective valve replacement surgery were studied. Patients who received somatostatin and trimetaphan in addition to sufentanil had significantly smaller serum growth hormone and plasma glucagon concentrations compared with those who received sufentanil and sodium nitroprusside. The cortisol response to surgery was inhibited in both groups. There were no significant differences in catecholamine concentrations between the two groups. There was no effect of the additional inhibition of glucagon and growth hormone on circulating concentrations of glucose and lactate, but plasma concentrations of non-esterified fatty acids increased significantly. Thus the addition of somatostatin increased the suppression of the hormonal response to cardiac surgery by sufentanil.
Site of action of fentanyl in inhibiting the pituitary-adrenal response to surgery in man.
To determine the site of action of fentanyl in attenuating the pituitary-adrenal response to surgery, we have measured serum concentrations of cortisol and growth hormone during and after a standardized surgical procedure in two groups of patients. One group received fentanyl 15 micrograms kg-1 i.v. immediately before the start of surgery; a second group received fentanyl 15 micrograms kg-1 i.v. together with corticotrophin releasing factor 100 micrograms i.v., growth hormone releasing hormone 100 micrograms i.v. and arginine vasopressin 10 units i.m. The concomitant administration of the releasing factors with the opioid resulted in a significantly greater serum concentration of cortisol 30, 60, 120 and 240 min after surgery commenced, compared with the group which received fentanyl alone. Similarly, the growth hormone response in the combined group was significantly greater than in the fentanyl-alone group 30 min after the start of surgery. We conclude that the inhibitory effect of fentanyl on surgically-induced secretion of pituitary hormone was mediated directly or indirectly via the hypothalamus.
Release of substance P from rat hypothalamus and pituitary by endothelin.
Endothelin-1 is a 21 amino acid peptide originally isolated from porcine aortic endothelium and has recently been localized within the central nervous system. We have administered endothelin-1 in a dynamic perfusion system in order to study its possible effects on the rat hypothalamus and anterior pituitary. Tissue (hypothalami or quartered pituitaries) was placed into plastic chambers and was perfused with oxygenated Krebs-bicarbonate solution. After an interval to establish stable basal peptide release, endothelin-1 was administered at two doses (0.1 and 1 microM) and the release of substance P, vasoactive intestinal peptide, 7B2, and somatostatin was measured, the last being detectable only in hypothalamic perfusates. Both concentrations of endothelin-1 led to a significant increase (P less than 0.01) in the release of substance P from the hypothalamus and pituitary, but not of vasoactive intestinal peptide, 7B2, or somatostatin. Thus after the 0.1 microM and 1 microM endothelin-1 perfusion substance P release from the hypothalamus increased by 125 +/- 5% and 215 +/- 15% (mean +/- SEM) of basal and from the pituitary by 168 +/- 8% and 276 +/- 15% (mean +/- SEM). No change occurred in the output of ACTH or other pituitary hormones. The release of substance P from hypothalamus or pituitary after stimulation with endothelin-1 was not blocked when a calcium free medium was used. Endothelin-1 binding sites were identified on rat pituitary cell membranes. These findings suggest the possibility that endothelin may act as a paracrine substance, neurotransmitter, or neuromodulator in the hypothalamo-pituitary axis.
Effects of a novel 21-amino steroid, U74006F, on the rat pituitary-adrenocortical axis.
The 21-amino steroid U74006F is a potent inhibitor of lipid peroxidation and has been shown to affect beneficially the acutely injured central nervous system. Therapeutically, it is desirable for this compound to be devoid of steroid side-effects. We have demonstrated a significant (P less than 0.001) inhibition of basal ACTH secretion from cultured rat pituitary cells during a 24-h incubation at concentrations (10-100 mumols/l) previously demonstrated to inhibit lipid peroxidation in vitro. U74006F also inhibited corticotrophin-releasing factor (CRF)-stimulated ACTH secretion significantly and the combination of dexamethasone and U74006F completely blocked CRF-41-stimulated ACTH secretion. Administration of U74006F in vivo (30 mg/kg, orally, every 6 h for 30 h) had no effect on ACTH levels in normal rats (84 +/- 38 vs 45 +/- 6 ng/l in control animals) but increased ACTH levels in adrenalectomized rats (1330 +/- 295 vs 464 +/- 79 ng/l in control animals, P less than 0.02). This increase in ACTH was not observed when adrenalectomized animals were maintained on the same regime of U74006F for 5 days. Our data suggest that U74006F is capable of exerting inhibitory effects on ACTH secretion in vitro. In vivo, effects on ACTH secretion were stimulatory rather than inhibitory and only occurred short-term in adrenalectomized animals or chronically in adrenalectomized rats maintained on dexamethasone. No effects on the pituitary-adrenocortical axis were seen following short-term or chronic administration of U74006F in normal rats.
Intracellular mechanisms involved in the stimulation of growth hormone release from rat anterior pituitary cells by Russell's viper venom.
We have previously shown that a heat-stable component of Russell's viper venom (RVV) releases GH in a dose-dependent manner from cultured rat anterior pituitary cells. We have now investigated the intracellular mechanisms involved in RVV-stimulated GH release by concomitant administration of RVV with known intracellular mediators in rat pituitary cells. 3-Isobutyl-1-methylxanthine (IBMX; 0.5 mmol/l), added to cultured rat anterior pituitary cells simultaneously with RVV, at concentrations up to a maximally effective dose of 10 micrograms/ml, increased GH release (3.7-fold, 4.0-fold and 2.0-fold; P less than 0.001) compared with the effect of venom alone. These effects were additive, indicating that RVV and IBMX stimulate through different intracellular messengers. RVV failed to increase the formation of basal or IBMX-stimulated intracellular cyclic AMP (cAMP), confirming that RVV affects GH release through a cAMP-independent pathway. 12-0-Tetradecanoylphorbol-13-acetate (TPA; 0.1 mumol/l), added simultaneously with various doses of RVV (0.1-10 micrograms/ml), did not increase GH release beyond the maximal effect of RVV. This result indicates that RVV might be stimulating GH release through a similar mechanism to that of TPA (by activating protein kinase C). When pituitary cells were perifused with Ca(2+)-free medium or verapamil (50 mumol/l), RVV-stimulated GH release was inhibited by 65 and 42% respectively. This reflects the recognized requirement of Ca2+ for secretory processes. However, RVV (10 micrograms/ml) had no significant effect on intracellular free Ca2+ concentrations as measured using the fluorescent Ca2+ probe quin-2.(ABSTRACT TRUNCATED AT 250 WORDS)
Local anaesthesia for cataract surgery.
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Burmese Russell's viper venom causes hormone release from rat pituitary cells in vitro.
Acute and chronic hypopituitarism is associated with severe envenoming by the Burmese Russell's viper. We have demonstrated that in vitro, Burmese Russell's viper venom (0.1-10 micrograms/ml) causes a dose-dependent release of GH, TSH and ACTH from dispersed rat anterior pituitary cells in culture. At 10 micrograms/ml, venom causes a significant increase in the release of GH (344%, P less than 0.001), TSH (168%, P less than 0.005) and ACTH (greater than 700%, P less than 0.001). We have also shown that the component (or components) responsible for this stimulatory effect is stable to heat (60 degrees C, 1 h) and mild trypsinization. Repeated addition of venom (1 microgram/ml) to pituitary cells in a perifusion column system demonstrated attenuation of GH release. This reduced response was not due to depletion of the GH pool since the pituitary cells were subsequently able to respond to both GH-releasing factor (GRF) stimulation and KCl depolarization. Somatostatin in a dose which abolished GRF-stimulated GH release failed to affect venom-stimulated GH release, implying that venom acts in a cyclic AMP-independent manner. We conclude that Burmese Russell's viper venom has direct effects on pituitary hormone release in vitro. Whether these effects contribute to its known actions in vivo on the function of the pituitary remains to be established.
Mechanisms involved in the effects of TRH on GHRH-stimulated growth hormone release from ovine and bovine pituitary cells.
Incubation of cultured ovine pituitary cells with growth hormone-releasing hormone (GHRH) (10(-12)-10(-7) M) stimulated growth hormone secretion up to 3-fold. At a maximal stimulatory concentration of GHRH (10(-10) M), thyrotropin-releasing hormone (TRH) (10(-7) M) caused an inhibition of growth hormone release to approx. 50% of the response obtained with GHRH alone (during a 15 min incubation period). TRH also caused a small inhibition of the GHRH-stimulated cellular cyclic AMP level but this effect was only significant at a relatively high concentration of GHRH (10(-9) M). Incubation of cultured bovine pituitary cells with GHRH (10(-11)-10(-8) M) plus TRH (10(-7) M) caused a significant stimulation of growth hormone release by up to 40%, compared with the response obtained with GHRH alone (at all concentrations of GHRH). TRH (10(-7) M) had no effect on GHRH (10(-8) M)-stimulated cellular cyclic AMP levels in a partially purified bovine pituitary cell preparation. The effects of varying extracellular [Ca2+] (0.1-10 mM) on intracellular [Ca2+] and on the responsiveness to releasing hormones were also determined using ovine pituitary cells. GHRH (10(-10) M)-stimulated growth hormone release was inhibited when cells were incubated at both high (10 mM) and low (0.1 mM) [Ca2+] (compared with 1 mM or 3 mM Ca2+) with or without TRH (10(-7) M). At 1 mM Ca2+, TRH produced a synergistic effect with GHRH to stimulate growth hormone release. However, at 3 mM Ca2+ TRH inhibited GHRH-stimulated growth hormone release.(ABSTRACT TRUNCATED AT 250 WORDS)
Use of quin 2 to measure calcium concentrations in ovine anterior pituitary cells and the effects of quin 2 on secretion of growth hormone and prolactin.
Intracellular free Ca2+ concentrations [Ca2+]i were measured in ovine anterior pituitary cells using the quin 2 technique. Thyrotropin-releasing hormone (TRH) increased, dopamine decreased and growth hormone-releasing hormone (GHRH) had no detectable effect on [Ca2+]i. Loading the cells with quin 2, at an intracellular concentration less than that used during calcium determination, reduced both basal growth hormone (GH) and (to a small extent) prolactin secretion. Loading cells with quin 2 also markedly reduced GHRH-stimulated GH secretion. However, TRH-stimulated prolactin secretion was 3-times basal irrespective of quin 2 loading. The results indicate that the use of quin 2 to measure [Ca2+]i in some cell types may be complicated by actions of quin 2 on cellular function.
Effects of dopamine and somatostatin on phorbol ester-stimulated prolactin and growth hormone secretion.
Incubation of cultured ovine pituitary cells with the tumor-promoting phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA) (0.1-100 nM), caused a dose-related stimulation of both growth hormone (ED50 approximately 4 nM) and prolactin (ED50 approximately 14 nM) secretion. Stimulation by TPA (100 nM) produced a substantial 10-fold increase in growth hormone with a smaller, 2-fold rise in prolactin secretion over 30 min; significant effects on the release of both hormones occurred within 2 min. Treatment with TPA also produced a small, time- and concentration-dependent rise in cellular cyclic AMP content which reached, at maximum, a level 20-30% over basal values. Non-tumor-promoting phorbol esters did not stimulate the secretion of either growth hormone or prolactin. In the presence of TPA (10 nM), dopamine (1-1000 nM) suppressed prolactin secretion to a level close to that observed for maximal inhibition of unstimulated cells. At high concentrations (0.1-1.0 microM) dopamine also partially attenuated (by 43%) the TPA-induced stimulation of growth hormone secretion. Somatostatin (0.01-1.0 microM) completely inhibited the substantial (approximately 9-fold) TPA-induced stimulation of growth hormone secretion (inhibitory ED50 approximately 47 nM), and also suppressed TPA-stimulated prolactin secretion to the control level. Our results suggest that activation of protein kinase-C may be involved in the stimulatory regulation of both growth hormone and prolactin secretion in sheep pituitary cells. Failure of TPA to attenuate the inhibitory activity of dopamine and somatostatin suggests that inhibitory regulation occurs at, or beyond, the point in the secretory process regulated by protein kinase-C.
Epidemic classical heat stroke: clinical characteristics and course of 28 patients.
Patients with classical heat stroke are different in many ways from those with exertional injury; contrasts included difference in demographic factors, prior general health, in-hospital complications and laboratory abnormalities (lactate, liver enzymes, pH, electrolytes). Severe hyperkalemia, acute renal failure, rhabdomyolysis and disseminated intravascular coagulation often dominate the course of patients with exertional heat stroke but are uncommon in those with classical heat stroke (Table 4). While lactic acidosis is the rule in exertional injury, it is somewhat unusual in patients with classical heat stroke and when above 3 mmoles/L predicts a poor outcome or death. In spite of the advanced age and multiple medical problems of the patients with classical heat stroke, careful attention to early and aggressive cooling and scrutiny for potential complications can result in salvage of most patients.
Oral clonidine loading in hypertensive urgencies.
The response to oral clonidine hydrochloride loading in 36 severely hypertensive patients is presented. Each patient initially received 0.2 mg of clonidine hydrochloride, followed by 0.1 mg each hour until a dose of 0.7 mg had been given, or the diastolic blood pressure (BP) reached a predetermined goal (110 mm Hg or total fall of at least 20 mm Hg). Only two patients (6%) failed to reach this goal. Supine BP in the group fell from 212 +/- 22 (SD)/139 +/- 11 (SD) mm Hg to 151 +/- 21 (SD) mm Hg at six hours. The average dose of clonidine required was 0.45 mg and control was maximized at five hours. The response to oral clonidine loading in the individual patient was not predictive of the eventual dose of clonidine necessary to achieve acceptable BP control at two weeks. Oral clonidine loading is safe and effective in the management of "hypertensive urgencies" and offers several advantages over parenteral antihypertensive agents in this clinical situation.
Clonidine overdose: report of six cases and review of the literature.
Clonidine poisoning usually causes depressed sensorium, hypotension, and bradycardia. Some patients manifest respiratory depression and miosis simulating narcotic overdose. Supportive care with judicious administration of intravenous fluids, occasionally supplemented by a dopamine infusion, usually reestablished adequate blood pressure. Tolazoline, an alpha-blocker, may reverse clonidine's effects should other efforts fail. Atropine should be used if bradycardia is hemodynamically significant. With massive overdose, clonidine's partial alpha-agonist properties may predominate, resulting in marked hypertension requiring cautious therapy. The experience at Parkland Memorial Hospital with clonidine overdose in six patients demonstrates the myriad of clinical presentations possible.
Alpha adrenergic blockade by prazosin in therapy of essential hypertension.
To elucidate the mechanism of action of prazosin in lowering the blood pressure, we studied the vascular pressor responsiveness to the infusion of norepinephrine and angiotensin II before and after prazosin in patients with essential hypertension. After prazosin, the pressor response to the alpha agonist norepinephrine was markedly attenuated, but the vascular reactivity to angiotensin II was unchanged. There was a threefold increment in the dose of norepinephrine required to elicit a 20 mm Hg rise in diastolic blood pressure after prazosin therapy. These results confirm that prazosin induces significant alpha adrenergic blockade during the treatment of essential hypertension.