Page 534 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

522 FLUID THERAPY

TABLE 21-1 Effects of Cardiovascular Drugs on Renal Function
Pharmacologic
Class Examples Mechanism of Action Effects on Renal Function

Angiotensin- Benazepril, enalapril, Inhibit converting enzyme, preventing Reduce the activity of the renin-
converting lisinopril, ramipril conversion of AT-1 to AT-2; also angiotensin-aldosterone system; can
enzyme inhibitors reduce degradation of vasodilator reduce intra-glomerular filtration
kinins pressure by blocking angiotensin
II-mediated vasoconstriction of the
efferent arteriole
Angiotensin Losartan, Block AT-1 receptors of angiotensin II As described for angiotensin-converting
receptor blockers candesartan enzyme inhibitors; may also affect
tissue renin-angiotensin-aldosterone
systems
Catecholamines Dobutamine, Stimulate b and a receptors to increase Increase renal perfusion pressure; dilate
dopamine cardiac output and blood pressure; low renal blood vessels (dopamine)
doses of dopamine stimulate
dopaminergic receptors in renal
arterioles
b-Adrenergic Metoprolol Blocks b-adrenoceptor Decrease renin
blockers Carvedilol Blocks b- and a-adrenoceptor Vasodilator effect
May reduce renal blood flow
(dose related)
Digitalis glycosides Digoxin Sensitize baroreceptors Reduce sympathetic nerve activity;
may reduce activation of the
renin-angiotensin system
Diuretics* Prevent reabsorption of electrolytes and Increase urine volume and urinary
water at various sites along the renal electrolyte loss; high dose can
tubules precipitate volume depletion and acute
Loop diuretics Furosemide (administered IV) can renal failure; IV furosemide may cause
(furosemide) release atrial natriuretic peptide and dilation of renal arterioles
prostaglandins
Thiazide diuretics
Potassium-sparing Blocks aldosterone receptor in the kidney
diuretics and in CV tissues (aldosterone)
(amiloride,
spironolactone,
eplerenone)
Human brain Nesiritide Increase cycle GMP Increase GFR, decreases RPF
natriuretic Probable dilation of afferent arteriole Increase sodium excretion and
peptide urine volume
Vasodilators Hydralazine, sodium Dilate systemic arterioles by diverse May increase renal perfusion; if
nitroprusside, mechanisms (e.g., generation of nitric hypotension develops renal blood
prazosin oxide; a-adrenergic blockade) flow can decrease

*Also see Table 21-2.

Diuretics may also reduce left ventricular afterload by antiinflammatory drugs (NSAIDs), or primary renal fail-
reducing sodium loading and vascular resistance in ure may reduce the renal delivery of a diuretic. In the case
arterioles. of renal failure, endogenous organic acids can compete
The clinical pharmacology of these drugs and effects with furosemide for transport across the proximal neph-
on renal function (Tables 21-1 and 21-2) are relevant ron. Once secreted into the filtrate, a diuretic inhibits salt
to understanding their effectiveness and limitations. All and water transport via a specific mechanism and at
of the commonly used diuretics, except spironolactone, relatively specific sites along the nephron. 72,112,142
are delivered by renal blood flow and secreted as organic Figure 21-5 demonstrates the general sites of action of
acids into the proximal tubule. Circulatory failure, the commonly used diuretics. The importance of under-
reduced renal blood flow, administration of nonsteroidal standing these details can be illustrated by two examples.