Page 413 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

380 SECTION | IV Drugs of Use and Abuse

VetBooks.ir (1,8-dihydroxyanthroquinone) is a synthetic anthraquinone animals, and water must be freely available. Other contra-
Osmotic cathartic use should be avoided in dehydrated
drug.
indications for use include congestive heart failure, gas-
Five percent of an oral dose of bisacodyl is absorbed.
There is some absorption of anthraquinone glycosides, and trointestinal stasis, and hepatic or renal impairment
delayed transit through the small intestine decreases effec- (Henninger and Horst, 1997; Ezri et al., 2006).
tiveness. Anthraquinones are secreted in the milk, causing Hyperosmotic cathartics draw water into the intestinal
clinical effects in the nursing young (Boothe, 2001). tract via osmosis. Intestinal distension promotes motility.
As suggested by the term “irritant laxative,” these Effects are usually evident 3 12 h after dosing in mono-
compounds cause contact irritation of gastrointestinal gastrics and within 18 h in ruminants.
mucosa and increase fluid secretion into the lumen. There is minimal slow absorption of most cathartics,
Diarrhea occurs 6 8 h after diphenylmethane cathartics but up to 20% of the magnesium in a dose of magnesium
are administered, and can be severe after large doses sulfate is absorbed in the small intestine (Henninger and
(Boothe, 2001). Pink discoloration of alkaline urine can Horst, 1997; Boothe, 2001). Renal failure enhances sys-
be used to confirm phenolphthalein ingestion. Acidic temic magnesium accumulation, leading eventually to
urine will turn pink with addition of sodium hydroxide or hypermagnesemia. Magnesium ions promote the release
sodium bicarbonate (Papich, 1990). of cholecystokinin, which promotes peristalsis. Systemic
Vegetable oil laxatives are hydrolyzed by lipase in the effects of magnesium include inhibition of calcium ion
small intestine and form sodium and potassium salts, release at neuromuscular junctions, inhibition of acetyl-
which act as soaps, producing irritation (Boothe, 2001). choline release, decreased sensitivity of motor endplates,
Ricinoleic acid is the most potent of these compounds, and decreased excitability of myocyte membranes, lead-
and initiates rapid and complete colonic emptying. This ing to paralysis. Clinical signs of hypermagnesemia sec-
clinical effect is seen 4 8 h after administration of castor ondary to magnesium sulfate administration in the horse
oil in small animals and 12 18 h postdosing in large ani- occurred within 6 h of magnesium sulfate administration
mals. Animals treated with castor oil should be fed moist, and included perspiration, progressive tremors, recumb-
bulky material afterward. ence, severe tachycardia, tachypnea, pale mucous mem-
Anthraquinones are hydrolyzed by bacteria in the large branes, prolonged capillary refill times, flaccid paralysis
intestine to emodins, which stimulate the myenteric of the head and neck, and loss of flexor and perineal
plexus. Anthraquinones produce catharsis after 6 12 h in reflexes, but horses remained alert.
small animals and 12 36 h in large animals (Boothe, Absorbed phosphate from phosphate-containing cath-
2001). Catharsis is accompanied by reduced hydration artics can deplete intracellular potassium ions and induce
and electrolyte loss. Abdominal pain or colic is produced hypokalemia. Dehydration and electrolyte imbalances are
by large doses. Changes in urine color have been reported the most common changes associated with cathartics
with anthraquinones. With chronic use of these laxatives, (Papich, 1990). Management is aimed at correcting these
the myenteric plexus degenerates, causing a loss in intes- imbalances. Calcium is used to treat hypermagnesemia.
tinal motility. Calcium ions displace magnesium ions from cell mem-
branes. Response to therapy is rapid, but repeated dosing
with calcium gluconate is sometimes needed. Diuresis
Hyperosmotic Cathartics with IV fluids and furosemide promote renal excretion.
Henninger and Horst (1997) reported that horses treated
Magnesium sulfate, or Epsom salt, is commonly used as
for hypermagnesemia had serum magnesium concentra-
an osmotic cathartic in 6% isotonic solution. Other
tions within reference ranges the next day. Electrolyte
magnesium-containing cathartics include magnesium
changes in asymptomatic patients given oral sodium phos-
hydroxide (milk of magnesia), magnesium oxide, and
phate for presurgical catharsis returned to normal within
magnesium citrate. Sodium sulfate (Glauber’s salt),
24 h (Ezri et al., 2006).
sodium phosphate, potassium sodium tartrate, and sodium
tartrate (Rochelle salt) are used as cathartics, and inges-
tion of large quantities of sodium chloride also produces Enemas
catharsis. The sugar alcohols mannitol and sorbitol, and An enema is a material given intrarectally to induce
synthetic disaccharides such as lactulose, are also used as defecation. Commonly used enemas include soft anionic
cathartics. Polyethylene glycol 3350 (PEG), once avail- soap in water, isotonic or hypertonic sodium chloride, sor-
able only with prescription for whole-bowel irrigation bitol, glycerol, sodium lauryl sulfate, sulfoacetate, mineral
before colonoscopy procedures and to treat body-packer oil, olive oil, and phosphate salts (Boothe, 2001).
patients (Farmer and Chan, 2003), is currently available Toxicosis has been reported in people and small ani-
over the counter (MiraLAX). mals administered hypertonic sodium phosphate solutions,