Effect of feeding on renin-angiotensin-aldosterone system of the horse

Abstract

The effect of feeding frequency and associated meal size on the renin-angiotensin-aldosterone system (RAAS) in seven horses was examined. A daily maintenance ration of hay-grain pellets was provided either as a multiple feeding regimen (MF), in which the ration was divided into six equal portions fed at 4-h intervals, or as a single large feeding (SF) given from 9 A.M. until 11 A.M. Plasma renin activity (PRA), aldosterone (PAC), cortisol (PCC), protein concentration (TP), packed cell volume (PCV), and serum sodium and potassium were measured serially. To prevent significant RAAS stimulation due to strenuous exercise or by assuming orthostatism after a period of recumbency, the horses were trained to stand in 1 X 4-m tie stalls during the experiments. Changes in Na intake were prevented by disallowing nonration salt sources. A 12:12 light-dark interval was maintained. During the MF experiment, only serum Na changed diurnally, with concentrations lowest in early morning and highest before midday. In contrast, during the SF experiment, PRA was increased at 0.5, 1.0, and 3.0 h and PAC was increased at 3.0, 5.0, and 7.0 h after onset of feeding (P less than 0.005). Increased TP and PCV suggested transient hypovolemia was responsible for renin release. Significant increases in Na and decreases in K occurred while eating; however, K increased postprandially to be coincident with aldosterone. Except for a transient increase during feeding in SF, PCC demonstrated a similar circadian rhythm in both experiments. It was concluded that 1) episodic feeding (SF) causes significant diurnal variation of the RAAS in the horse, and 2) spontaneous circadian activity of the RAAS cannot be demonstrated in this species during a steady-state feeding regimen (MF).

Chronobiology and Pharmacologic Modulation of the Renin–Angiotensin–Aldosterone System in Dogs: What Have We Learned?

“…Our results are consistent with previous investigations in dogs , horses (Clarke et al 1978(Clarke et al , 1988, and humans (Cugini et al 1981(Cugini et al , 1985, which underlines the similarity of body fluid homeostasis in mammals. A cosine model with a fixed 24-h period was found to fit the periodic variations of RA, BP, urinary aldosterone, and potassium excretion well, as suggested by the quality of the model diagnostics.…”

“…Studies have been performed under (i) sodium restriction (Cugini et al 1981(Cugini et al , 1985, (ii) episodic vs. continuous feeding (Blair-West and Brook 1969;Clarke et al 1978Clarke et al , 1988, and (iii) fasting conditions (Cugini et al 1987). In contrast, the impact of timed feeding on the chronobiology of the renin cascade remains controversial.…”

Effects of feeding large amounts of grain on colonic contents and feces in horses

“…However, a transient increase in PCV and plasma protein concentration lasting < 3 hours after ingestion of large amounts of grain has been observed in other studies. [17][18][19][35][36][37] There may be 2 reasons why postprandial dehydration was not observed in the present study. First, blood samples were only collected 5.5 and 11.5 hours after a large amount of grain was offered, whereas in other studies, [17][18][19][35][36][37] a postprandial increase in PCV and plasma protein concentration lasted < 3 hours.…”

“…In horses, experimental administration of 1 or 2 meals/d that are 1-hour in duration induces a state of transient dehydration with activation of the renin-angiotensin-aldosterone system. [16][17][18][19] On the basis of those findings, it was proposed that feeding a few large meals could cause postprandial dehydration of colonic contents and lead to impaction of the large colon, which could initiate other forms of colic such as large colon displacement and volvulus. 3 However, this theory was determined by use of artificial feeding regimens that are not used for domestic horses.…”

Tissue kallikrein permits early renal adaptation to potassium load

“…To study the ability of mice to adapt to a high K + diet, TK −/− and TK +/+ mice were equilibrated for a few days on the standard diet (0.8% K + ) and then switched to a "high" K + (2% K + ) diet (Institut National de la Recherche Agronomique, Jouy en Josas, France). To maximize feedingdependent influences on plasma K + level (39), TK −/− and TK +/+ mice were supplied with their daily ration, which they were allowed to eat over a 4-h period (between 10:00 AM and 2:00 PM). For convenience, the 12:12 light/dark cycle was inverted.…”