[Effects of secretin on renal functions in the dog]

Dual effect of secretin on nephron filtration and proximal reabsorption depending on the route of administration

Secretin is a vasoactive peptide capable of acting on transmembrane volume fluxes. We measured nephron filtration (SNGFR) and resorption during secretin microinjection (MIJ) into the tubular lumen or microperfusion (MP) into peritubular capillaries. In 24 rat nephrons, SNGFR, measured by collections from the distal tubule, rose from 25+/- 4 to 61+/-8 nl/min during MIJ of saline containing secretin 10(-9) M into the last convolution of the proximal tubule (LP). Percent and absolute resorptions rose from 70 to 90% and from 20+/-4 to 56+/-8 nl/min, respectively. During MIJ of secretin, 3 x 10(-)(9) M into the first convolution of the proximal tubule, SNGFR, measured at LP, rose from 32+/-4 to 61+/-8 nl/min, percent and absolute reabsorptions from 52+/-4 to 78+/-3% and from 16+/-2 to 50+/-7 nl/min, respectively (n = 30). During MP of secretin, 1.5x10(-9) M, SNGFR fell from 39+/-6 to 15+/-4, resorption from 19+/-4 to 9+/-2 nl/min, while percent resorption rose from 43+/-6 to 59+/-5% (n = 15). While all MIJ and MP changes were significant (P<0.001), paired pre- versus post-MIJ and MP values were not. Secretin is a powerful vasoconstrictor when perfused into peritubular capillary blood, unlike systemic and intra-arterial injections. When injected into the tubular lumen, it up-regulates SNGFR and increases reabsorption directly.

Effect of kallikrein-kinin system on calcium-vitamin D3 metabolism

Kallikrein has been reported to stimulate callus formation and cell proliferation. It is well-known that vitamin D3 play an role on the metabolism of calcium. However, the relation between vitamin D3 and kallikrein are still poorly understood. We have studied the effect of kallikrein on calcium and vitamin D3 with canine kidney. Mongrel dogs were anesthetized with sodium pentobarbital and prepared by the method of Nakanishi et al. Drugs were infused into renal artery for 60 minutes under three different serum calcium concentrations (Exp. I, Exp. II, Exp. III). Blood samples were collected at the midpoint of the 60 minutes urine collection period. Plasma vitamin D3 concentrations were measured by the metabolites of 25-hydroxyvitamin D3 and 1 alpha,25-dihydroxyvitamin D3. 25-hydroxyvitamin D3 was estimated by competitive protein binding assay and 1 alpha,25-dihydroxyvitamin D3 was determined by radioreceptor assay after separated using HPLC. Electrolytes of sodium, potassium, chloride, calcium and phosphorus in serum and urine, plasma cyclic AMP, glomerular filtration rate, renal blood flow and blood pressure were also measured respectively. Plasma 1 alpha,25-dihydroxyvitamin D3 concentration was found to be decreased by the infusion of kallikrein (0.02 KU/kg/min) in three experimental conditions, especially in Exp. II. Bradykinin (0.02 microgram/kg/min) also caused to kallikrein-like changes of vitamin D3. It is suggested that kallikrein-kinin system is related to inhibit the mechanism of vitamin D3 activation system on kidney.

A microsphere study on the effects of somatostatin and secretin on regional blood flow in anesthetized dogs

The endogenous peptides somatostatin and secretin are effective in the therapy of upper gastrointestinal tract bleeding and acute pancreatitis. The clinical effects may be partly brought about by changes in the regional blood flow. To evaluate the effects of somatostatin (50 and 100 micrograms/min over 6-8 min) and secretin (0.1 and 0.5 U X kg-1 X min-1 over 3-5 min) on tissue blood flow, particularly of the gastrointestinal tract, the tracer microsphere reference sample method was used in anesthetized dogs. Infusion of somatostatin significantly diminished gastric and pancreatic blood flow whereas no changes of duodenal and ileal blood flow could be obtained. Blood flow through spleen, kidneys and adrenal glands was increased but no changes were observed in the blood flow of other tissues. Cardiac hemodynamics remained unchanged. Secretin increased the blood flow of the duodenum, the kidneys and the adrenal glands and diminished gastric blood flow without changing pancreatic, ileal, hepatic, pulmonary and muscle blood flow. Cerebral, pituitary and myocardial blood flow was increased by a higher dose of secretin. It also evoked a slight but significant positive ino- and chronotropic effect. Since secretin and somatostatin differ in their respective effects on gastrointestinal blood flow it is suggested that the previously reported beneficial effects of both peptides on upper gastrointestinal bleeding cannot solely be attributed to changes in regional blood flow.

Effect of glucagon on renin secretion in the dog

The effects of glucagon alone or in combination with theophylline on renin section were studied in relation to renal hemodynamic responses in anesthetized dogs. The intrarenal infusion of glucagon (0.5 microgram/kg/min) increased heart rate, renal blood flow, glomerular filtration rate and urine flow without any effect on renin secretion, but at a higher dose (1.0 microgram/kg/min) it increased renin secretion significantly. Theophylline (0.1 mg/kg/min) did not affect renal hemodynamics but caused a slight increase in renin secretion after 30--60 min infusion. The combined infusion of glucagon (0.5 microgram/kg/min) with theophylline (0.1 mg/kg/min) increased renin secretion markedly, although it produced renal hemodynamic changes similar to those induced by glucagon alone. These effects were not suppressed by d,l-propranolol (1.0 microgram/kg/min). It is suggested that the increase in renin secretion caused by the combined infusion of glucagon and theophylline resulted mainly from an increase in cyclic AMP in the juxtaglomerular cells, and not from stimulation of beta-adrenoceptors.