Caffeine is pharmacologically similar to the other xanthine drugs, such as
theobromine and theophylline; however, these three agents differ in the intensity
of their actions on various structures. Caffeine's CNS and skeletal muscle effects
are greater than those of the other xanthines. In all other areas, theophylline
has greater activity than caffeine, although some studies report that caffeine
has greater diuretic effect than theobromine. The increased levels of intracellular
cyclic-AMP mediate most of caffeine's pharmacologic actions. Caffeine competitively
inhibits phosphodiesterase, the enzyme that degrades cyclic 3'- 5' adenosine
monophosphate. Caffeine stimulates all levels of the CNS. Caffeine's cortical
effects are milder and of shorter duration than those of the amphetamines. In
slightly larger doses, caffeine stimulates medullary vagal, vasomotor and respiratory
centers, promoting bradycardia, vasoconstriction, and increased respiratory
rate. Caffeine produces a positive inotropic effect of the myocardium and a
positive chronotropic effect at the sinoatrial node, causing transient increases
in heart rate, force of contraction, cardiac output and heart work. doses greater
than 250 mg, the centrally mediated vagal effects of caffeine may be masked
by increased sinus rates; tachycardia, extrasystoles, or other major ventricular
arrhythmias may result.
Caffeine constricts cerebral vasculature. In contrast, the drug directly dilates
peripheral blood vessels, decreasing peripheral vascular resistance. The effect
of this decrease in peripheral vascular resistance (and possibly that of vagal
cardiac stimulation) on blood pressure is offset by increased cardiac output
(and possibly stimulation of the medullary vasomotor area). The overall effect
of caffeine on heart rate and blood pressure depends on whether CNS or peripheral
effects predominate. Therapeutic doses of caffeine increase blood pressure only
slightly.
Caffeine stimulates voluntary skeletal muscle, increasing the force of contraction
and decreasing muscular fatigue. The drug also stimulates gastric acid secretion
from parietal cells. Caffeine increases renal blood flow and glomerular filtration
rate and decreases proximal tubular reabsorption of sodium and water, resulting
in mild diuresis. Caffeine stimulates glycogenolysis and lipolysis, but increase
in blood glucose and in plasma lipids are insignificant in normal patients.
Tolerance may develop to the diuretic, cardiovascular, and CNS effects of caffeine.
Pharmacokinetics
Caffeine is rapidly distributed throughout the body tissues, readily crossing
the placenta and blood-brain barrier. Approximately 17% of the drug is bound
to plasma proteins. Caffeine has approximately a half-life (T½) 3-4 hours in
adults. In adults, the drug is rapidly metabolized in the liver to 1-methyluric
acid, 1-methylxanthine and 7-methylxanthine. Caffeine and its metabolites are
excreted primarily by the kidneys.
Last updated on RxList: 9/30/2008