Depression of proximal tubular sodium reabsorption in the dog in response to renal beta adrenergic stimulation by isoproterenol.

Water diuresis was produced in anesthetized hypophysectomized, cortisone-treated dogs by infusion of 2.5% dextrose. Alpha adrenergic blockade of the left kidney produced by infusion of phenoxybenzamine in the left renal artery was associated with a significantly (P < 0.05) greater rate of urine flow (V) and free water excretion (C(H2O)) in the left kidney than in the right despite similar glomerular filtration rates (GFR) (17 +/- 1.3 ml/min, left; 18 +/-0.9 ml/min, right). Sodium excretion (U(Na)V) was similar in the two kidneys (3 and 5 muEq/min). When beta adrenergic stimulation of the left kidney was superimposed on alpha blockade by the addition of isoproterenol to the left renal artery infusate, GFR remained unchanged and similar in the two kidneys, as V and C(H2O) increased significantly (P < 0.01) in the left kidney but not in the right. When isoproterenol was discontinued, V and C(H2O) returned towards control in the left kidney and remained unchanged in the right. The ratios of the left kidney to the right during control, isoproterenol, and postcontrol were 1.22, 1.65, and 1.35, respectively, for V and 1.36, 1.90, and 1.44, respectively, for C(H2O). Sodium excretion remained unchanged and similar in the two kidneys throughout the study. The results indicate that blockade of alpha adrenergic activity inhibits the increased proximal tubular sodium reabsorption which anesthesia induces in the dog. Beta adrenergic stimulation appears to decrease proximal tubular sodium reabsorption but does not prevent virtually complete reabsorption of the increased quantity of delivered sodium by the ascending limb of the loop of Henle and the distal tubule. These changes in sodium reabsorption presumably are not associated with changes in colloid osmotic pressure or hydrostatic pressure in the peritubular capillary inasmuch as cortical and non-cortical plasma flow, filtration fraction, and mean arterial pressure in the left kidney were unchanged. Thus, isoproterenol probably produced its effects through a direct action on the renal tubule, possibly through the mediation of the adenyl cyclase system.

Renal effects of adenosine 3',5'-cyclic monophosphate and dibutyryl adenosine 3',5'-cyclic monophosphate. Evidence for a role for adenosine 3',5'-cyclic monophosphate in the regulation of proximal tubular sodium reabsorption.

Stable water diuresis was produced in anesthetized, hydrocortisone-treated hypophysectomized dogs by infusion of 2.5% dextrose. Infusion of adenosine 3',5'-cyclic monophosphate (cyclic AMP) in the left renal artery decreased ipsilaterally glomerular filtration rate (GFR), cortical and non-cortical renal plasma flow, and tended to increase urine flow (V) and free-water clearance (C(H2O)) despite a decrease in mean arterial pressure. Infusion of dibutyryl adenosine 3',5'-cyclic monophosphate (dibutyryl cyclic AMP) in the left renal artery increased V and C(H2O) significantly (P<0.01) bilaterally with essentially no change in GFR, in total renal plasma flow or its cortical and non-cortical components. For each kidney the magnitude of the change in V was similar to the magnitude of the change in C(H2O) and the change in sodium excretion was trivial. Cyclic AMP probably produced its effects on renal hemodynamics and mean arterial pressure wholly or in part through the action of metabolites such as 5'-AMP and adenosine on the renal and systemic vasculature. The absence of an effect of dibutyryl cyclic AMP on renal hemodynamics and its bilateral effect may be explained by the resistance of this nucleotide derivative to metabolism. Dibutyryl cyclic AMP appears to decrease by direct cellular effect(s) proximal tubular sodium reabsorption but does not prevent virtually complete reabsorption of the increased load of sodium in the distal nephron. This effect on the kidney is qualitatively and quantitatively similar to the effect of renal arterial infusion of isoproterenol. The results suggest that synthesis of cyclic AMP in proximal renal tubule cells in response to stimulation of beta adrenergic or other receptors can mediate a decrease in the proximal tubular reabsorption of sodium.

Role of the sympathetic nervous system in the renal response to hemorrhage.

In 12 studies, a femoral artery and vein of a donor dog treated with desoxycorticosterone were connected by tubing to a renal artery and vein of a recipient dog treated with desoxycorticosterone, and the kidney with its nerve supply intact was perfused at femoral arterial pressure. Infusion of normal saline, which contained albumin, from 2.7 to 3.1 g/100 ml, in the donor produced significant natriuresis in a kidney of the donor (from 112 to 532 muEq/min) and in the perfused kidney (from 60 to 301 muEq/min) of the recipient. Increased sodium excretion in the perfused kidney was associated with an increase in the clearances of inulin and para-aminohippurate (P < 0.01) and a decrease in hematocrit of perfusing blood (P < 0.01). Infusion was continued in the donor while recipient was bled 23 ml/kg, with a decrease in mean arterial pressure from 152 to 130 mm Hg. Sodium excretion in perfused kidney decreased from 301 to 142 muEq/min (P < 0.01), whereas sodium excretion in donor was unchanged (506 VS. 532 muEq/min; P > 0.3). Clearance of inulin by perfused kidney was not significantly affected by bleeding (26 +/-SE 2 VS. 25 +/-SE 2; P > 0.2), but the clearance of para-aminohippurate was decreased by bleeding (P < 0.01), so that filtration fraction increased. As the perfused kidney of the recipient dog continued to receive blood from the natriuretic donor dog when the recipient dog was bled, the decrease in sodium excretion that bleeding produced in the perfused kidney was presumably mediated by renal nerves. Thus, an increase in nervous stimuli to the kidney that is not sufficient to decrease glomerular filtration rate can increase the tubular reabsorption of sodium and thereby significantly decrease its excretion. This property of the sympathetic nervous system to affect tubular reabsorption of sodium suggests that an increase in sympathetic activity may constitute an important mechanism for the renal conservation of sodium when intravascular volume is contracted by hemorrhage or other cause.

Sites of action of sodium depletion on aldosterone biosynthesis in the dog.

Secretion of cortisol, corticosterone, and aldosterone was measured in vivo in normal and sodium-depleted hypophysectomized dogs. Biogenesis of steroids was then measured in vitro with outer slices of the adrenals of the same dogs. In some studies, metyrapone or puromycin was added. In vivo, sodium depletion stimulated the production of cortisol, corticosterone, and aldosterone. In vitro, tissues from sodium-depleted animals released more aldosterone, but less corticosterone than those from sodium-replete controls. The results are interpreted to indicate that (a) biosynthesis of aldosterone is regulated at at least two sites in the biosynthetic pathway. The final conversion, that of corticosterone to aldosterone, is stimulated by sodium depletion. This effect persists for at least 3 hr while slices from sodium-depleted dogs are incubated in vitro. Stimulation at this site is thus relatively stable in vitro; its activation by sodium depletion is not inhibited by puromycin in the dog. Stimulation at this site can explain, at least in part, the increased effectiveness of adrenocorticotropin (ACTH) on aldosterone biogenesis during sodium depletion.(b) the earlier site at which sodium depletion stimulates the secretion of aldosterone is "above" the position of desoxycorticosterone in the pathway; it is probably at the conversion of cholesterol to pregnenolone. Stimulation at this site is quickly lost during incubation of adrenal slices. It is thus relatively unstable in vitro; its activation by sodium depletion is inhibited by puromycin in the dog.

Effect of Escherichia coli on fluid transport across canine small bowel. Mechanism and time-course with enterotoxin and whole bacterial cells.

An Escherichia coli strain isolated from a patient with severe cholera-like diarrhea elaborates a partly heat-labile enterotoxin shown to cause prompt adenyl cyclase stimulation and isotonic fluid secretion by canine jejunum. Both responses disappear upon removal of the enterotoxin. The duration of action of a submaximal dose of this E. coli enterotoxin was brief, despite sustained exposure to the jejunum, suggesting inactivation of the enterotoxin by its interaction with the mucosa. Inoculation of whole bacterial cultures of this E. coli strain into canine duodenum was followed by bacterial survival and induction of net secretion after 4-7 h. The onset of fluid production was associated with increasing gut mucosal adenyl cyclase activity. Washed bacterial cells could also produce fluid secretion. In vivo multiplication of this enterotoxin-producing E. coli was demonstrated 6-12 h after intraduodenal inoculation of approximately 10(6) organisms. This was associated with fluid secretion. Intestinal fluid production occurred without microscopic pathology in the mucosa.

Assessment and validation of animal models to evaluate topical effects of substances on gastrointestinal mucosa.

The in situ rabbit colon model is a sensitive and reproducible test to evaluate the topical effect of up to three substances applied to the colonic mucosa. Vibra-Tabs (doxycycline hyclate), Inderal (propranolol hydrochloride), and Slow-K (potassium chloride) were compared for topical effects in the Carlborg-Densert cat esophagus model, the Alphin-Droppleman cat gastric mucosa model adapted for dog intestine, and the rabbit colon. Because results were comparable in all models, additional dosage formulations were subsequently tested only in the rabbit colon model. After exposure of the tissue to drugs, macroscopic and histologic effects were scored on four- and eight-point scales, respectively. In all three models, Vibra-Tabs and Inderal produced the highest macroscopic and histologic scores, although Slow-K was also irritating. In the rabbit colon model, potassium released from Slow-K and Micro-K Extencaps caused more irritation than from controlled-release GITS (KCl) (gastrointestinal therapeutic system KCl).