Postprandial changes of renal blood flow. Studies on conscious dogs on a high and low sodium intake

Uncomplicated human type 2 diabetes is associated with meal-induced blood pressure lowering and cardiac output increase

AIMS

Since many type 2 diabetes patients experience postprandial hypotension, the aim of this study was to unravel meal-related changes in systemic hemodynamics and autonomic nervous system (ANS)-balance.

METHODS

Forty-two age-matched males (15 type 2 diabetes; 12 metabolic syndrome; 15 controls) without overt autonomic neuropathy received a standardized high-fat mixed meal after an overnight fast. Hemodynamic variables were measured by finger plethysmography. Fourier analysis was used to calculate the low-/high-frequency (LF/HF)-ratio, a marker of autonomic nervous system-balance, and baroreceptor reflex sensitivity (BRS).

RESULTS

Following the meal, diastolic blood pressure (DBP) decreased in type 2 diabetes patients only, paralleled by a significant decrement in systemic vascular resistance (SVR) and an increase in cardiac index. All groups showed an increase in postprandial heart rate. Controls, but not metabolic syndrome or type 2 diabetes patients, showed a meal-related increase in LF/HF-ratio. When combining all study subjects, homeostatic model assessment-insulin resistance (HOMA-IR) was inversely correlated with changes in DBP, SVR, LF/HF-ratio and BRS.

CONCLUSIONS

Based on these data, we hypothesize that in patients with uncomplicated type 2 diabetes, insulin resistance hampers adequate meal-induced sympathetic activation, leading to a decrease in SVR and resulting in a postprandial drop in DBP.

Intracoronary gastrin 17 increases cardiac perfusion and function through autonomic nervous system, CCK receptors, and nitric oxide in anesthetized pigs

The release of gastrointestinal hormones has been reported to modulate reflex cardiovascular responses caused by gastric distension, although the role played by gastrin 17 is as yet unknown. The present study was therefore planned to determine the primary in vivo effect of gastrin 17 on coronary blood flow and cardiac function and the involvement of autonomic nervous system, CCK1/2 receptors, and nitric oxide (NO). In 40 anesthetized pigs, gastrin 17 was infused into the left anterior descending coronary artery at constant heart rate and arterial blood pressure. In 35 of the 40 pigs, the mechanisms of the observed hemodynamic responses were analyzed by repeating gastrin 17 infusion after autonomic nervous system and NO blockade, and after specific CCK receptors agonists/antagonists administration. Intracoronary gastrin 17 administration caused dose-related increases of both coronary blood flow and cardiac function. The intracoronary co-administration of CCK33/pentagastrin and gastrin 17 potentiated the coronary effects observed when the above agents were given alone ( P <0.05). The potentiation of the cardiac response was observed only with the co-administration of pentagastrin and gastrin 17 ( P <0.05). Moreover, blockade of muscarinic cholinoceptors (intravenous atropine) and of α-adrenoceptors (intravenous phentolamine) did not abolish the hemodynamic responses to gastrin 17. The cardiac and vascular effects of the hormone were prevented by blockade of β-adrenoceptors (intravenous atenolol and butoxamine), CCK1/2 receptors (intracoronary lorglumide and CAM-1028), and NO synthase (intracoronary Nω-nitro-l-arginine methyl ester). In conclusion, gastrin 17 primarily increased coronary blood flow and cardiac function through the involvement of CCK receptors, β-adrenoceptors, and NO release.

Cushing's syndrome, glucocorticoids and the kidney

Glucocorticoids (GCs) affect renal development and function in fetal and mature kidneys both indirectly, by influencing the cardiovascular system, and directly, by their effects on glomerular and tubular function. Excess GCs due to endogenous GC overproduction in Cushing's syndrome or exogenous GC administration plays a pivotal role in hypertension and causes increased cardiac output, total peripheral resistance and renal blood flow. Glucocorticoids increase renal vascular resistance (RVR) in some species and experimental settings and decrease RVR in others. Short term administration of adrenocorticotrophic hormone or GCs causes an increased glomerular filtration rate (GFR) in humans, rats, sheep and dogs. Interestingly, chronic exposure may cause a decreased GFR in combination with a higher cardiovascular risk in human patients with Cushing's syndrome. Glomerular dysfunction leads to proteinuria and albuminuria in canine and human Cushing's patients, and some cases also show histological evidence of glomerulosclerosis. Tubular dysfunction is reflected by an impaired urinary concentrating ability and disturbed electrolyte handling, which can potentially result in increased sodium reabsorption, hypercalciuria and urolithiasis. Conversely, chronic kidney disease can also alter GC metabolism. More research needs to be performed to further evaluate the renal consequences of Cushing's syndrome because of its implications for therapeutic aspects as well as the general well-being of the patient. Because there is a high incidence of Cushing's syndrome in canines, which is similar to the syndrome in humans, dogs are an interesting animal model to investigate the link between hypercortisolism and renal function.

Renal effects of gastrin C-terminal tetrapeptide (as pentagastrin) and cholecystokinin octapeptide in conscious rabbit and man

Pentagastrin and cholecystokinin octapeptide (CCK8) were infused i.v. at three different doses in two sets of 4 conscious rabbits following a repeated measurements design (130, 1,300 and 13,000 pmol kg-1 min-1 pentagastrin; 5, 50 and 450 pmol kg-1 min-1 CCK8). In man, two different doses of pentagastrin (13 and 65 pmol kg-1 min-1) were infused in two groups of 6 subjects, and CCK8 (2 pmol kg-1 min-1) in a third group. According to published human postprandial levels, plasma CCK8-like immunoreactivity concentrations were supraphysiological at all doses infused. In the rabbit, pentagastrin produced a dose-related fall in urine flow and free water clearance, but no significant change in systemic and renal haemodynamics, electrolyte excretion and measured plasma constituents; however, in human subjects, pentagastrin increased renal sodium excretion and reduced potassium excretion but did not change glomerular filtration rate. In the rabbit, CCK8 produced a dose-related fall in plasma renin activity, plasma calcium concentration and mean arterial blood pressure; dose-dependent increases in effective renal plasma flow, glomerular filtration rate and renal sodium excretion. In man, changes in sodium and potassium excretion similar to pentagastrin were observed; there were no significant changes in plasma renin activity, plasma calcium concentration, blood pressure, effective renal plasma flow or glomerular filtration rate. The pharmacological renal effects of pentagastrin in conscious water-loaded rabbits resemble vasopressin. In contrast, CCK8's most striking effect was vasodilatation and was unusual in inhibiting rather than stimulating renin release. In man the net changes in urine composition found during infusion of these peptides are similar to those produced by the potassium-sparing diuretics, amiloride and triamterene. However the generally weak renal effects observed, even at pharmacological doses, indicate that these peptides are unlikely to influence renal function under normal physiological conditions.

Protein-mediated elevations in renal hemodynamics: existence of a hepato-renal axis?

Unlike the intestinal hyperemia which occurs following ingestion of a mixed meal, postprandial renal hemodynamic responses to food appear to be specific to protein-rich meals. Several observations have led to the proposal that following ingestion of protein (meat), a blood-borne factor(s) is released into the systemic circulation which elicits the increases in renal hemodynamics. Glucagon was initially considered as a prime hormonal candidate since the polypeptide is preferentially secreted by the endocrine pancreas in response to protein-rich meals and because glucagon elevates renal hemodynamics by selectively dilating renal afferent arterioles. However, recent data indicate that at postprandial blood levels, glucagon fails to directly mediate protein-induced renal hyperemia and hyperfiltration thus questioning the physiologic importance of glucagon on the renal vasculature. Data have accumulated indicating the importance of the liver and hepatic metabolism during protein-mediated elevations in renal hemodynamics. A hypothesis is advanced suggesting the existence and physiologic importance of a "hepato-renal axis" in mediating the postprandial increases in renal hemodynamics following ingestion of protein-rich meals. The blood-borne factor(s) mediating this response remains to be further clarified and defined.

Left atrial pressure and postprandial diuresis in conscious dogs on a high sodium intake

5 conscious, well trained, female dogs kept on a high sodium intake (14 meq Na/kg bw) were used to measure left atrial pressure (LAP), urine volume (V), sodium and potassium excretion (UNaV, UKV) as well as plasma osmolality (Posm) before and up to 180 min after food intake. The dogs were fitted with a catheter in the left atrium (thoracotomy). In all experiments (n=23) LAP increased postprandially (pp) above fasting controls. The mean peak increase ranged from 4 to 6 cm H2O and was observed as early as 61-80 and as late as 161-180 min pp. Increase in LAP was closely correlated to V which rose from 36+28 to 160+51/ul/min. kg. pp V was also correlated to pp UNa V, which increased from 4.8 +/- 3.3 to 34.O+/-8.5/ueq/min-kg. The pp increase in LAP and its close relation to pp V and pp UNav emphasize the assumption that intrathoracic receptors are involved in the regulation of body fluids.