Systemic and centrally mediated angiotensin II effects in the horse

The relation of maternal fluid balance to offspring passive immunity

The objectives were to measure changes in fluid balance of mares at parturition and relate those changes to the foals' acquisition of passive immunity. Twelve Thoroughbred mares and their foals were observed for suckling behavior for the first 12h post-partum. The mare's water intake, packed cell volume, plasma protein concentration, and plasma osmolality were measured pre- and post-partum. The foals' immunoglobulin concentrations were measured at birth and 24h later. The median increase from pre- to post-foaling in mares' packed cell volume was 4.0% (interquartile range=0.3-6.8). The median increases in plasma protein and plasma osmolality after foaling were 0.9 (0.5-1.4)g/dL and 2.5 (0.8-6.0) milliosm/kg, respectively. Median daily water intake was 13.8 (7.6-20.8)L before and 42.1 (26.2-61.4)L 24h after foaling. The foals' IgG at 24h was significantly correlated with their dam's water intake in the 12h following parturition (r=0.66, P=0.039). The median duration of suckling during the first 4h of the foals' lives was 20 (5-85)min and 85 (42-220)min during the first 12h. The median number of suckling bouts was 34 (11-71) during the first 12h after birth.

Water homeostasis and diabetes insipidus in horses

Diabetes insipidus (DI) is a rare disorder of horses characterized by profound polyuria and polydipsia (PU/PD), which can be caused by loss of production of arginine vasopressin (AVP). This condition is termed neurogenic or central DI. DI may also develop with absence or loss of AVP receptors or activity on the basolateral membrane of collecting-duct epithelial cells. This condition is termed nephrogenic DI. Equine clinicians may differentiate true DI from more common causes of PU/PD by a systematic diagnostic approach. DI may not be a correctable disorder, and supportive care of affected horses requires an adequate water source.

Effect of oral administration of electrolyte pastes on rehydration of horses

OBJECTIVE

To determine whether the composition of electrolyte pastes formulated for oral administration influences voluntary water intake (WI) by horses recovering from furosemide-induced dehydration.

ANIMALS

6 horses.

PROCEDURES

Voluntary WI, body weight, and blood and urine constituents were measured before and after induction of dehydration by furosemide administration and overnight withholding of water; these same variables also were measured during a 36-hour rehydration period. Each horse was evaluated 4 times with random application of 4 treatments (electrolyte pastes) that provided 0.5 g of KCl/kg of body weight, 0.5 g of NaCl/kg, 0.25 g of NaCl and 0.25 g of KCl/kg, or no electrolytes (control treatment). Electrolyte pastes were administered 3 times (4, 8, and 12 hours after start of the rehydration period).

RESULTS

Administration of all electrolyte pastes resulted in significantly greater voluntarily WI, compared with the control treatment, and was accompanied by significantly greater recovery of body weight when NaCl was a component of the paste. Administration of NaCl and NaCl-KCl pastes tended to produce a state of transient hyperhydration; however, electrolyte administration also resulted in significantly greater urine production and electrolyte excretion during the final 24 hours of the rehydration period. Adverse effects of oral administration of hypertonic electrolyte pastes were not observed.

CONCLUSIONS AND CLINICAL RELEVANCE

Oral administration of electrolyte pastes to dehydrated horses increases voluntary WI and improves rehydration during the rehydration period. Rehydration is more rapid and complete when NaCl is a component of the electrolyte paste.

Vasopressin in dehydrated and rehydrated ponies

Six pony mares deprived of water for 24 hours showed significant increases in plasma vasopressin (2.8 pg/ml) and osmolality (9 mosmol/kg). When water was made available the ponies drank rapidly (5 of 6 drank to satiety within 90 seconds) and corrected their fluid deficits precisely. Vasopressin did not return to predehydration levels until osmolality did after 15 minutes of access to water. The horse differs from rodents and humans, but is similar to pigs in that vasopressin levels do not fall before osmolality returns to normal. Oropharyngeal factors, therefore, may not be as important in vasopressin release in horses as in other species.