ACUTE MANNITOL AND SALINE VOLUME EXPANSION IN THE RAT: EFFECT ON TRANSEPITHELIAL POTENTIAL DIFFERENCE IN PROXIMAL TUBULES

1. Transepithelial potential difference (PDte) of proximal tubules was measured in rats under control conditions (C), and mannitol-saline and saline extracellular fluid volume expansion (MVE, SVE, respectively) under conditions of normal net lumen to basal sodium transport. 2. PDte was measured in kidneys bathed with Hartmann's solution or covered with mineral oil under both volume-expanded conditions together with their controls. 3. PDte was significantly lower in kidneys bathed with Hartmann's solution than those covered with oil. 4. In MVE rats, with mineral oil covering the kidneys, PDte (expressed as mean and s.e.m.) was for the control 2.20 +/- 0.05 (n = 45) mV and MVE 1.97 +/- 0.04 (n = 36) mV, lumen positive, a significant reduction of 10% (P less than 0.001). In SVE rats, with mineral oil covering the kidneys, PDte was for C = 2.42 +/- 0.05 (n = 74) mV and SVE = 1.93 +/- 0.03 (n = 67) mV, a significant reduction (P less than 0.001) of 20%. 5. According to thermodynamic considerations, neither of these changes is sufficient to explain the 50% inhibition of Na transport measured previously during MVE and SVE with autologous tubular fluid. The present results offer further evidence supporting the idea that the inhibition of Na transport during MVE and SVE is largely due to inhibition of the active Na transporting step.     

EFFECT OF ATRIAL NATRIURETIC PEPTIDE ON CELLULAR ELEMENT CONCENTRATIONS IN RAT PROXIMAL TUBULES: EVIDENCE FOR INHIBITION OF THE SODIUM PUMP

1. In order to further define the action of atrial natriuretic peptide (ANP) on proximal tubular (PT) transport, combined clearance and electron microprobe X-ray (EMPX) experiments were performed on five male Wistar rats infused with ANP (0.16 nmol/kg per h) and nine control animals. 2. Electron microprobe X-ray analysis of PT cell electrolytes (mmol/ kg wet weight) revealed a similar [Na]_i in both the control and ANP treated groups (16.4 ± 0.4 vs 16.5 ± 0.4; P= 0.894). [Cl]_i was lower in the ANP treated animals (14.8 ± 0.3 vs 12.0 ± 0.3; P<0. 0001) as was [K]_i (131.4 ± 1.4 vs 114 ± 1.7; P<0.0001). The PT cells in the ANP treated group had a significant reduction in dry weight (20.1 ± 0.3 g%vs 19.0 ± 0.3 g%; P<0.024), indicating significant cell swelling. Thus, despite a normal [Na]_i, there was net accumulation of Na_i following ANP treatment. 3. These results are consistent with accumulation of Nai due to inhibition of the Na pump followed by cell swelling and subsequent regulatory volume decrease with exit of K and Cl. These results are the first to show the effect of ANP on PT intracellular electrolytes.     

EFFECTS OF SODIUM DEPLETION ON TISSUE CONCENTRATIONS OF THE NATRIURETIC HORMONE VASOACTIVE INTESTINAL PEPTIDE

1. Variations in dietary sodium intake have been shown to affect the plasma concentration, the metabolic clearance rate and secretion rate of vasoactive intestinal peptide (VIP). In this study we sought to determine the effect of sodium depletion on the concentration of VIP in plasma and in three tissues, namely heart, lung and kidney. 2. Male Sprague-Dawley rats were placed on low or normal sodium diets and drinking water ad libitum. A third group was placed on a low salt diet and in addition were given frusemide, 1 mg/kg per day in the drinking water. After 7 days the rats were killed, a blood sample collected and tissues harvested. VIP concentrations were determined by radio-immunoassay on unextracted plasma and in tissue after extraction. 3. There were significant differences between the three groups in the Concentration of VIP in the lung (P< 0.0005), kidney (P< 0.005) and plasma (P< 0.025) but not the heart. In the group that received frusemide and the low sodium diet, VIP in the lung was significantly lower than the low sodium (P< 0.005) and normal sodium (P< 0.0001) groups. Similar differences were noted in the kidney (frusemide vs low sodium, P< 0.001; frusemide vs normal, P< 0.01) and plasma (frusemide vs low sodium P< 0.001, frusemide vs normal P< 0.05). 4. We conclude that sodium depletion decreases the concentration of VIP in plasma and in its metabolizing tissues.     

NORADRENALINE RELEASE FROM THE RAT HEART DURING ANOXIA: EFFECTS OF CHANGES IN EXTRACELLULAR SODIUM CONCENTRATION AND INHIBITION OF SODIUM UPTAKE MECHANISMS

1. Anoxic perfusion of the isolated rat heart releases noradrenaline in the absence of sympathetic nerve fibre stimulation. 2. Anoxic noradrenaline release is enhanced by reducing the extracellular Na+ concentration, consistent with the proposal that such release occurs by carrier-mediated efflux. 3. Release is also enhanced by lignocaine but inhibited by amiloride and ethylisopropylamiloride, suggesting that sodium entry into adrenergic nerve terminals during anoxia occurs by Na+/H+ (and possibly Na+/Ca2+) exchange.     

EFFECTS OF HYDROXOCOBALAMIN AND HAEMOGLOBIN ON NO-MEDIATED RELAXATIONS IN THE RAT ANOCOCCYGEUS MUSCLE

1. The effects of hydroxocobalamin (Vitamin B_12a) on relaxations produced by nitric oxide (NO), some NO-donating compounds and nitrergic nerve stimulation in isolated preparations of the rat anococcygeus muscle were compared with the effects of haemoglobin. 2. Hydroxocobalamin (30 μmol/L) significantly reduced relaxations induced by NO (0.1–3 μmol/L) and sodium nitroprusside (SNP; 0.01–0.3 μmol/L) but did not affect relaxations induced by glyceryl trinitrate (GTN; 0.01–1 μmol/L), S-nitrosocysteine (0.1–0.3 μmol/L) or stimulation of nitrergic nerves. A higher concentration of hydroxocobalamin (100 μmol/L) slightly reduced nitrergic nerve stimulation-induced relaxations. 3. Haemoglobin (10 μmol/L) blocked relaxation induced by NO and reduced relaxations induced by SNP, GTN, S-nitrosocysteine and nitrergic nerve stimulation. 4. When nitrergic nerve stimulation-induced relaxations had been partially reduced by the NO synthase inhibitor l-NAME (5–10 μmol/L), hydroxocobalamin had only a weak and transient inhibitory effect. 5. Noradrenergic contractions induced by field stimulation were not affected by hydroxocobalamin (30 μmol/L), but were enhanced by haemoglobin (10 μmol/L). 6. The results suggest that the transmitter released from nitrergic nerves in anococcygeus muscles resembles NO-releasing compounds such as S-nitrosocysteine and GTN but not SNP or free NO.     

THE PERMEABILITY OF COLLECTING DUCTS TO 22Na+ AND 36CI− IN RAT ISOLATED PAPILLAE

1. The diffusional permeability of collecting ducts to ²²Na⁺ and ³⁶CI^? was measured in rat papillae in vitro. 2. The permeability of the collecting duct to ³⁶CI^? was 0.72 (s.e.m. = 0.01; n= 356) μn/sec which was significantly higher than the value of 0.51 (s.e.m. =0.01; n= 356) μm/sec measured for ²²Na⁺. 3. Collecting ducts in papillae taken from rats on a high sodium intake had a ²²Na⁺ permeability of 0.63 (s.e.m. = 0.04; n= 53) μm/sec which was significantly higher than the value on a normal salt intake (0.50, s.e.m. =0.04; n= 46 μm/sec). 4. When papillae from normal rats were studied in plasma taken from salt loaded rats, the ²²Na⁺ permeability of 0.59 (s.e.m. =0.04; n= 18) μm/sec was significantly higher than when incubated in plasma from normal rats (0.44, s.e.m. =0.05; n= 12) μm/sec. 5. An extract of urine with natriuretic activity had no effect on ²²Na⁺ permeability when tested in this system. 6. Adrenalectomy, PGE₂, indomethacin and antidiuretic hormone had no significant effect on ²²Na⁺ and ³⁶CI^? permeability.     

COMPARISON OF THE ATTENUATING EFFECTS OF FOUR β-ADRENOCEPTOR AGONISTS ON RAT ISOLATED UTERUS AND AORTA

1. The ability of four 8-adrenoceptor agonists to attenuate oxytocin (0.2,2 and 20 nmol/L) or KCI (20,40 and 80 mmol/L)-induced Contractions of the uterus (n= 5–8 for each agonist) and the KCI (18 mmol/L)-induced contractions of the aorta (n = 9 for each agonist) from rats, pretreated with oestradiol has been compared. 2. Isoprenaline, salbutamol, terbutaline and procaterol (0.1–10μmol/L) attenuated the contractions of the uterus and the aorta. All four agonists had similar attenuating potencies on the uterus. 3. Procaterol caused the same maximal attenuation (33%) on the aorta as the other β-adrenoceptor agonists and is thus acting as a full β₂-adrenoceptor agonist under these experimental conditions. Isoprenaline and procaterol were much more potent than salbutamol and terbutaline in attenuating the aorta responses. 4. This study showed that isoprenaline and procaterol were potent attenuants on both the uterus and aorta whereas salbutamol and terbutaline were potent uterine but only modest aorta attenuants. This preliminary study indicates that the responsiveness of uterine and vascular tissue to certain β₂-adrenoceptors differs.     

METABOLIC STUDIES OF URIDINE IN RATS WITH DOCA-SALT HYPERTENSION AND ON HIGH SODIUM DIET

1. The steady-state metabolic clearance and calculated secretion rate of the pyrimidine nucleoside uridine were studied by equilibrium infusion in normal rats, rats on a high sodium diet, rats made hypertensive by subcutaneous injection of deoxycorticosterone acetate (DOCA), unilateral nephrectomy and high sodium drinking fluid, and two control groups of rats for the hypertensive group. 2. Basal plasma uridine concentration in DOCA-salt hypertension rats was found to be significantly reduced to 3.99 ± 0.31 pmol/L (mean ± s.e.m.) compared with control rats (11.98 ±1.64 μmol/L). Metabolic clearance (MCR) in DOCA-salt hypertensive rats was significantly raised (200.54±10.77 mL/kg per min) compared with control rats (65.17 ± 1.99 mL/kg per min). No difference was found in plasma uridine concentration and MCR among the other two control groups and high sodium diet rats. Calculated secretion rate was unchanged in all animals. No significant differences were found between different groups of rats in blood pressure responses to uridine. 3. The raised metabolic clearance and reduced plasma uridine concentration in DOCA-salt hypertension may be consistent with increased intracellular transport and phosphorylation of uridine to the physiologically active compound uridine monophosphate (UMP) which would lead to arteriolar constriction, hypertension and natriuresis. The results contrast with those in humans with extracellular fluid (ECF) expansion from endstage renal failure and rats with one-kidney, one-clip (1K1C) hypertension but are not due to the pharmacological effects of deoxycorticosterone. The difference may be due to the haemodynamic consequences of reduced renal perfusion pressure or reduced renal mass compared with DOCA-salt model.     

THE EFFECTS OF PERTUSSIS TOXIN ON VASOCONSTRICTOR AND VASODILATOR AGENTS IN THE PITHED RAT MAY NOT BE AN INDICATOR OF G-PROTEIN RECEPTOR COUPLING

1. In the present study, rats were treated with pertussis toxin (8.4 μg, i.v.) to determine whether pertussis toxin-sensitive G-proteins were linked to receptors that mediate effects in the cardiovascular system. 2. In the pithed rat the pressor responses to the α-adrenoceptor agonists noradrenaline and (to a lesser extent) methoxamine were attenuated by pertussis toxin treatment; the tachycardia produced by noradrenaline was unaffected. The pressor response to serotonin was also attenuated by pertussis toxin treatment. These observations are consistent with known effects of pertussis toxin on these receptors. 3. The vasodepressor responses to the muscarinic receptor agonist carbachol and to adenosine were reduced by pertussis toxin, suggesting that these events may have been mediated by pertussis toxin-sensitive G-proteins. However, the depressor responses to the β₂-adrenoceptor agonist salbutamol and to the receptor-independent vasodilator drugs sodium nitroprusside and hydralazine were also reduced by pertussis toxin. These latter effects suggest that caution must be used in interpreting the effects of pertussis toxin since the mechanism of action of these drugs as elucidated in vitro is thought not to involve pertussis toxin-sensitive G-proteins.     

THE EFFECTS OF MAZINDOL AND 46-034 (SANDOZ) ON GLUCOSE OXIDATION AND INSULIN BINDING BY RAT ISOLATED FAT CELLS

1. The anorexic agent mazindol and its major metabolite 46-034 (Sandoz) in high concentrations (>0·4 mM) abolished basal and insulin-stimulated conversion of 1-¹⁴C-glucose to ¹⁴CO₂ by rat isolated fat cells. 2. High concentrations (1 mM) also inhibited specific binding of ¹²⁵I-insulin to fat cells. 3. The observed effects appeared to be due in part to perturbation of the plasma membrane since there was a rise in the lactate dehydrogenase content of the incubation medium, increased ¹²⁵I-insulin degradation and a reduction in cellular tritiated water space. 4. These effects are unlikely to be relevant to the therapeutic action of mazindol.     

DIFFERENTIAL BLOCKING EFFECTS OF PRAZOSIN AND YOHIMBINE ON PRESSOR RESPONSES TO NEURONALLY RELEASED AND EXOGENOUSLY ADMINISTERED NORADRENALINE IN THE PITHED RAT

The effects of prazosin and yohimbine on pressor responses to sympathetic nerve stimulation and intravenous injections of noradrenaline, phenylephrine and clonidine were examined in pithed rats to determine the postjunctional location of alpha 2-adrenoceptors in the vascular smooth muscle. Prazosin antagonized the pressor responses to phenylephrine and to sympathetic nerve stimulation more effectively than the responses to noradrenaline and to clonidine. Yohimbine antagonized the pressor responses to noradrenaline and to clonidine more effectively than the responses to sympathetic nerve stimulation and to phenylephrine. These results suggest that alpha 2-adrenoceptors as well as alpha 1-adrenoceptors produce vasoconstriction in the rat vasculature and support the hypothesis that alpha 1-adrenoceptors are predominantly located within the neuroeffector junction in contrast to an extrajunctional location of alpha 2-adrenoceptors.     

EFFECT OF LIGNOCAINE ON CORONARY BLOOD FLOW, SYSTOLIC MYOCARDIAL FUNCTION AND MYOCARDIAL HIGH ENERGY PHOSPHATE STORES IN SWINE

1. To investigate the effect of lignocaine upon coronary blood flow, myocardial systolic wall function and high energy phosphate stores, lignocaine was administered as a rapid intravenous injection to 14 open chest anaesthetized swine. 2. Before and after injection, measurements were made of coronary blood flow by electromagnetic flow probe, per cent wall thickening by ultrasonic crystals, adenosine triphosphate (ATP) and creatine phosphate (CP) content by myocardial biopsy, and arterial pressure by central aortic catheter. The animals were divided into two groups based on whether or not they received a continuous low-dose infusion of lignocaine prior to the study. Group I received the continuous low-dose infusion of lignocaine and group II did not. 3. With a 2 mg/kg lignocaine injection, peak diastolic coronary flow rose significantly in groups I and II by 27 +/- 7 and 29 +/- 7% respectively. This was followed by a significant decline in per cent wall thickening in groups I and II of -11 +/- 2 and -19 +/- 6% respectively. In group I myocardial CP content decreased after lignocaine injection by 58 +/- 6% and ATP tended to rise even though systolic and diastolic pressure did not change significantly. In group II neither CP nor ATP changed significantly, but systolic and diastolic blood pressure decreased significantly. 4. Repeat lignocaine injections were given over a wider dosage range (0.5-4.0 mg/kg) to determine dose-response for lignocaine versus coronary blood flow. Coronary blood flow increased and per cent wall thickening decreased as doses of lignocaine were increased. 5. It was concluded that rapid intravenous lignocaine injection appeared to cause a dose-dependent coronary dilatation and systolic dysfunction. Pre-treatment with low-dose continuous infusion of lignocaine appeared to result in a decrease in CP and a rise in ATP when compared with no pre-treatment--despite a similar effect on myocardial function and coronary blood flow.     

THE EFFECTS OF SODIUM CHLORIDE, UREA AND MANNITOL ON THE PERMEABILITY IN VITRO OF RAT PAPILLARY COLLECTING DUCTS TO THO, 14C-UREA AND 22Na

1. The diffusional permeabilities of collecting duct membranes to THO, 14C-urea and 22Na+ have been measured at different concentrations of urea, NaCl and mannitol. 2. In the absence of urea in perfusate and bath or in its presence in low concentrations, the diffusional permeability to urea was 2.0 (s.e.m. = 0.15, n = 58) micrometer s-1, compared with 0.87 (s.e.m. = 0.06, n = 29) microgram s-1 when 200 mmol/l urea was present. The permeability of the collecting ducts to THO or Na+ was not affected by the different urea concentrations. 3. High concentrations of sodium chloride increased the diffusional permeability of collecting ducts to water and urea but did not affect the diffusional permeability of the collecting duct to Na+. 4. Mannitol had effects similar to those of sodium chloride. 5. In all media tested there was an increase in THO and urea permeability when supramaximal amounts of antidiuretic hormone were added. The increases in the various media for each substance were similar, despite widely different starting permeabilities. 6. The results suggest that solutes and water move across collecting duct epithelium by several pathways that respond differently to various stimuli.