John Laycock

The role of vasopressin in blood pressure regulation immediately following acute haemorrhage in the rat

1. The possible pressor effect of vasopressin immediately after acute haemorrhage has been studied using anaesthetized Brattleboro rats with diabetes insipidus and rats of the Long Evans parent strain.

Significance of an index of insulin resistance on admission in non-diabetic patients with acute coronary syndromes

BACKGROUND Insulin resistance is associated with ischaemic heart disease and has been proposed as a risk factor for subsequent myocardial infarction. AIM To investigate the potential use of a recently proposed insulin resistance index in identifying insulin resistance in patients admitted with an acute coronary syndrome. METHODS Single centre study of 441 non-diabetic patients admitted with chest pain to a coronary care unit and followed prospectively for a median of three years for outcome. Admission glucose and insulin concentrations were measured and from these values an admission index of insulin resistance (AIRI) calculated. Its association with other known factors in the insulin resistance syndrome, and subsequent outcome, was examined. RESULTS The AIRI was greater in patients with myocardial infarction than in a control group without myocardial infarction (p < 0.0001). A Cox regression model for subsequent cardiac death identified previous myocardial infarction (p < 0.0001), infarct size (p < 0.0001), and AIRI (p = 0.0033) as positive risk predictors. Patients of Indian subcontinent ethnic origin had greater AIRI values than white patients: mean (SD) 7.5 (1.3) v 4.6 (0.2), p < 0.001. CONCLUSIONS A simple index of insulin resistance measured on patients admitted with myocardial infarction provides an important predictive measure of poor outcome and is superior to admission glucose measurement. It may be useful in identifying patients admitted with myocardial infarction who could benefit from alternative early management strategies.

SEPARATE RECEPTORS MEDIATE OXYTOCIN AND VASOPRESSIN STIMULATION OF cAMP IN RAT INNER MEDULLARY COLLECTING DUCT CELLS

The two neurohypophysial hormones arginine vasopressin (AVP) and oxytocin have actions in the inner medullary collecting duct (IMCD) where both peptides induce an increase in cAMP accumulation. The present study has employed a novel IMCD cell line to determine whether these two hormones induce cAMP accumulation via common or separate receptors, and to characterize the potential receptors responsible. Equal volumes of vehicle (150 mM NaCl) or hormone/antagonist solutions were added to aliquots of 104 IMCD cells in the presence of 10‐3 m 3‐isobutylmethylxanthine (IBMX) and incubated at 37 °C for 4 min. cAMP levels were determined by radioimmunoassay and protein concentration by Bradford assay. Both AVP and oxytocin elicited dose‐dependent increases in cAMP generation, though oxytocin was less potent than AVP (EC50 = 1.6 × 10‐8 M vs. 7.4 × 10‐10 m). AVP at 10‐8 m and oxytocin at 10‐6 m, concentrations sufficient to elicit near‐maximal cAMP accumulation, resulted in cAMP levels of 73.4 ±1.7 and 69.0 ± 3.3 pmol (mg protein)−1 (4 min)−1, respectively (n = 10), compared with the vehicle‐treated basal value of 37.7 ± 2.2 pmol (mg protein)−1 (4 min)−1 (P < 0.001, n = 10). Combined AVP (10‐8 m) and oxytocin (10‐6 m) resulted in cAMP accumulation of 63.8 ± 3.1 pmol (mg protein)−1 (4 min)−1 (n = 10), which was not significantly different from the effect of oxytocin alone, but slightly less than that for AVP alone (P < 0.05). A submaximal concentration of AVP (10‐10 m) induced cAMP accumulation of 48.6 ± 2.5 pmol (mg protein)−1 (4 min)−1 (P < 0.01 compared with basal level of 34.9 ± 2.4 pmol (mg protein)−1 (4 min)−1, n = 10), which was blocked in the presence of a vasopressin V2 receptor antagonist (10‐7 m OPC‐31260) but not by the oxytocin receptor antagonist (10‐6 m [Pen1,pMePhe2,Thr4,Orn8]oxytocin) (36.3 ± 6.1 and 45.1 ± 1.3 pmol (mg protein)−1 (4 min)−1 respectively, P < 0.05, n = 10). A submaximal concentration of oxytocin (10‐7 m) induced a cAMP accumulation of 45.8 ± 1.8 pmol (mg protein)−1 (4 min)−1 (n = 10), which was reduced by addition of 10‐6 m oxytocin antagonist (36.3 ± 2.1 pmol (mg protein)−1 (4 min)−1, P < 0.05, n = 10), whereas co‐incubation with 10‐6 m of the V2 receptor antagonist had no effect (43.2 ± 1.3 pmol (mg protein)−1 (4 min)−1, n = 10). These results indicate that AVP and oxytocin induce cAMP accumulation from a common ATP pool in IMCD cells, and that separate vasopressin V2 and oxytocin receptor systems are involved, perhaps coupled to a common adenylate cyclase system.

Hypothalamic and plasma total nitrate/nitrite concentrations in spontaneously hypertensive rats

Stable end‐products of nitric oxide (NO) metabolism, nitrates and nitrites, were measured in hypothalamic extracts and plasma samples of Okamoto spontaneously hypertensive (SH) rats. The mean total nitrate/nitrite concentration was significantly lower in the hypothalami of SH rats compared with the normotensive Wistar Kyoto (WKY) control animals (P < 0.01). In contrast, their plasma concentrations were significantly higher (P < 0.05). These results indicate that the hypertensive state in SH rats is associated with a diminished production of hypothalamic NO, while the raised plasma nitrate/nitrite levels could reflect an increased compensatory endothelial NO synthase activity in these animals compared with the WKY controls.

The abnormal quinine drinking aversion in the Brattleboro rat with diabetes insipidus is reversed by the vasopressin agonist DDAVP: A possible role for vasopressin in the motivation to drink

The Brattleboro rat with hypothalamic diabetes insipidus (BDI) has an abnormal aversion to drinking quinine-adulterated water compared with normal rats of the parent Long Evans (LE) strain. This BDI animal tolerates marked hypovolemia and decreased body weight in preference to drinking the quinine-adulterated fluid, indicative of a reduced motivation to drink. Acute or chronic treatment of BDI rats with desamino-8D arginine vasopressin (DDAVP) restored to normal their drinking response to quinine solution. Partial restoration of fluid turnover in BDI rats with hydrochlorothiazide, which has an antidiuretic effect in diabetes insipidus (when vasopressin is absent), failed to abolish the abnormal drinking response to quinine-adulterated solution in 8 out of 12 animals. In contrast, induction of diabetes mellitus in LE rats, which resulted in a marked polydipsia and polyuria even though vasopressin was still present, did not impair the drinking response to quinine solutions. These results suggest that the abnormal drinking response to quinine-adulterated fluid in BDI rats is reversed by treatment with the vasopressin V2-receptor agonist DDAVP but is unlikely to be a consequence of the restoration of fluid turnover to normal levels by a renal action. A possible central action involving vasopressin and the motivation to drink is discussed.

A role for vasopressin in the stress-induced inhibition of gonadotrophin secretion: studies in the brattleboro rat.

The effects of stress on the secretion of adrenocorticotrophin, corticosterone and luteinizing hormone (LH) in rats congenially lacking hypothalamic vasopressin (Brattleboro rats) and in normal controls of the parent strain (Long Evans) have been compared in an attempt to examine the role of vasopressin in the stress‐induced depression of gonadotrophin secretion. In the Long Evans rats, stress (0.6 mg/100g histamine, ip) initiated, within 5 and 20 min respectively, significant (P <0.01, Students t‐test) increases in the plasma adrenocorticotrophin and corticosterone concentrations. It also caused a reduction in the serum LH concentration which was maximal at 5 min. By contrast, in the vasopressin deficient Brattleboro rats, stress had no effect on the serum LH concentration and produced only modest increases in pituitary adrenocortical activity compared with those in Long Evans controls. Pretreatment of both Long Evans and Brattleboro rats with dexamethasone (20μg/100 g ip, daily for 3 days) effectively abolished the pituitary‐adrenal response to stress. The steroid treatment also prevented the stress‐induced suppression of LH in the Long Evans rats; indeed, these animals, unlike the vehicle‐treated controls, exhibited a rise in serum LH concentration within 5 min of exposure to stress. Stress did not affect the serum LH concentrations in steroid‐treated Brattleboro rats. The results confirm previous reports that vasopressin is required for the full expression of the pituitary‐adrenocortical response stress. They also provide novel evidence which suggests that vasopressin released in stress contributes to the impairment of gonadotrophin secretion.

The effect of vasopressin on extracellular cation concentrations and muscle resting potentials in the rat.

1. The Na+ and K+ concentrations in plasma and cerebrospinal (c.s.f.), resting potentials in skeletal muscle fibres, cardiac beat to beat intervals and 90% repolarization times were measured in Long Evans rats (parent strain controls) and in Brattleboro rats with hypothalamic diabetes insipidus (DI). 2. Cation concentration measurements confirmed previous observations that Brattleboro DI rats are mildly hypokalaemic compared with rats of the parent Long Evans strain, and indicated that the c.s.f. [Na+] is significantly raised in the former group of animals while the [K+] in the c.s.f. samples is similar in the two groups. 3. The mean resting potential of deep skeletal muscle fibres in Brattleboro DI rats was significantly more negative than the corresponding value in the Long Evans rats, and this finding was in close agreement with the difference observed for the calculated K+ equilibrium potentials in the two groups of animals. 4. The beat to beat intervals and the 90% repolarization times of cardiac action potentials were also determined in Brattleboro DI and Long Evans rats, and the mean values for both variables were significantly shorter in the former group of animals. 5. The administration of Pitressin by subcutaneous injection (500‐100 mu./24 hr) to Brattleboro rats abolished the hypokalaemia and the hyperpolarization of skeletal muscle fibre membranes but had no significant effect on c.s.f. cation concentrations. 6. The present findings suggest that the absence of vasopressin in the Brattleboro DI rats results in a hyperpolarization of muscle cell membranes, and in changes in the cardiac action potential. These effects may be partly related to the mild hypokalaemia present in these animals.

Stability of the vasopressin V2 receptor-adenylyl cyclase system in rat kidney

In the Brattleboro rat with diabetes insipidus vasopressin V2 receptor mRNA and the mRNA of various adenylyl cyclase (AC) isoforms are moderately reduced compared with those of normal rats. In the present study renal vasopressin V2 receptor mRNA was modestly higher (by 34%), as was expression of AC 5, 6 and 9 mRNAs (up to 22% greater), in BDI rats treated with the vasopressin V2 receptor agonist desamino-[Arg8] vasopressin than in untreated controls. AC 4 mRNA was decreased by 17% following desamino-[Arg8s] vasopressin treatment. While the stimulatory Gsalpha mRNA was little affected by the desamino-[Arg8] vasopressin treatment, two of the inhibitory G proteins were raised (Galphai-2 by 54% and Galphai-3 by 57%). Treatment of Sprague-Dawley rats with a specific vasopressin V2 receptor antagonist (SR 121463A) was not associated with any marked changes in mRNA expression. These results indicate that the vasopressin V2 receptor adenylyl cyclase system mediating the antidiuretic response to vasopressin is relatively stable. The Gi proteins may be involved in the stabilizing mechanism.

Desamino-8d-arginine vasopressin treatment of Brattleboro rats: Effect on sensitivity to pressor hormones

The cardiovascular effects of intravenous injections of vasopressin, angiotensin II and noradrenaline were studied in anaesthetized adult male Brattleboro rats with hereditary diabetes insipidus on lifelong treatment with the vasopressin V2 receptor agonist desamino-8D-arginine vasopressin in the drinking fluid, which restored fluid input and output to normal rat values. The pressor response to 20 ng.kg-1 vasopressin was significantly greater (P < 0.005) in the vasopressin V2 receptor agonist-treated rats than in the control animals, but the responses to all higher doses of the peptide were comparable. Doses of noradrenaline from 40 to 160 ng.kg-1 had similar pressor effects in the treated and control rats, while the pressor response to the highest dose of noradrenaline (320 ng.kg-1) was significantly lower (P < 0.01) in the vasopressin V2 receptor agonist-treated rats. Furthermore the pressor responses to all three doses of angiotensin II (40, 80 and 160 ng.kg-1) were significantly attenuated in the treated rats compared to the control group (P < 0.001, P < 0.05 and P < 0.0005 respectively), as were the decreases in heart rate (P < 0.005 at 40 ng.kg-1, P < 0.01 at 80 ng.kg-1). The hypovolaemic stimulus induced by a blood loss of 20 ml.kg-1 resulted in a lower mean arterial blood pressure initially in the treated Brattleboro rats, but subsequent recovery was similar in both treated and control groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Opening doors and minds: a path for widening access

Background:  Students from disadvantaged socio‐economic backgrounds are under‐represented in UK medical schools. Many successful interventions are also highly labour‐intensive for medical schools to implement. We describe and evaluate a sustainable, low‐cost strategy that provides participants with targeted support, advice and experience.