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).

Summary Bacterial endotoxin injected intravenously into conscious ponies produced alterations in cardiopulmonary and gastrointestinal function. Specifically, tachypnoea, dyspnoea, hypoxaemia, colic, lactic acidosis and diarrhoea resulted from administration of 10μg/kg Escherichia coli endotoxin. Pretreatment of the ponies with a potent prostaglandin synthetase inhibitor, flunixin meglumine, prevented these ill effects of endotoxin. Résumé Des endotoxines bactériennes injectées par voie intraveineuse à des poneys conscients, engendrèrent des altérations des fonctions pulmonaires et gastro intestinales. En particulier de la tachypnée, de la dyspnée, de l'hypoxie, des coliques, de l'acidose lactique et de la diarrhée résultèrent de l'administration d'endotoxines colibacillaires. Le traitement préalable des poneys avec un puissant inhibiteur de la synthèse des prostaglandines, la flunixin méglumine, empêcha ces effets néfastes de l'endotoxine. Zusammenfassung Intravenös gegebenes, bakterielles Endotoxin verursachte cardiopulmonäre und gastrointestinale Funktionsstörungen bei Ponies. Eine Dosis von 10 μg Coli‐Endotoxin pro kg Körpergewicht rief Tachypnoe, Dyspnoe, Hypoxaemie, Kolik, Lactacidose und Diarrhoe hervor. Eine vorangehende Behandlung der Ponies mit einem wirksamen Prostaglandin‐Synthesehemmer (Flunixin Meglumin) vermochte die Endotoxinwirkung zu unterdrücken.

Clenbuterol, a bronchospasmolytic agent (beta 2 agonist) was studied in terms of its hemodynamic and airflow response in eight, healthy horses. Four animals were instrumented to record intrapleural pressure and air flow, these were used to compute pulmonary resistance, peak flow rates, and tidal volumes. Four animals were instrumented to record pulmonary arterial pressure, carotid arterial pressure, cardiac output, and arterial gas tensions. After control values were recorded, clenbuterol (0.8 microgram/kg) was intravenously administered to each horse in each experiment group. Following clenbuterol administration, non-elastic resistance of the lung or pulmonary resistance significantly decreased, 33.6% reduction at 10 min post-clenbuterol. Pulmonary resistance remained lowered during the entire procedure and showed no tendency of returning toward control values by 3 h post-clenbuterol. Within 30 sec following clenbuterol injection carotid arterial pressure decreased (mean pressure decrease 28.2%). Accompanying the change in arterial pressure, the heart rate drastically increased, 99.0%. Both changes were transient and returned to control ranges within 2 min. Clenbuterol appears to be effective in reducing non-elastic resistance of the lung, however intravenous administration to an animal with pre-existing cardiovascular or cardiopulmonary disease should be avoided.

Summary Six adult ponies were trained calmly to assume and maintain left lateral recumbency without the use of sedative or immobilising agents. During a 30 min recumbent period, pHa, arterial oxygen and carbon dioxide tensions (Pao2 and PacO2) and heart and respiratory rates were monitored at regular intervals to evaluate ventilatory response. Overall, there were no statistically significant differences found between mean control and recumbent or final standing values. When light‐weight ponies were compared to heavyweight ponies, only mean Pao2 at 10 mins recumbency was different. This information supports the hypothesis that ventilation impairment during the first 30 mins of equine general anaesthesia is primarily drug‐mediated, rather than initiated by recumbency per se.