K. Debrah et al., EFFECT OF ACUTE AND CHRONIC CAFFEINE USE ON THE CEREBROVASCULAR, CARDIOVASCULAR AND HORMONAL RESPONSES TO ORTHOSTASIS IN HEALTHY-VOLUNTEERS, Clinical science, 89(5), 1995, pp. 475-480
1. The effects of acute and chronic caffeine ingestion on supine- and
tilt- (60 min at 70 degrees) induced changes in middle cerebral artery
velocity (V-mca), heart rate, blood pressure and counter-regulatory h
ormone levels (catecholamines, growth hormone and cortisol) were studi
ed in nine healthy volunteers. A double-blind, placebo-controlled desi
gn was used to study acute effects followed by an open study after 6 d
ays of chronic caffeine use. 2. In the supine position, acute ingestio
n of caffeine (250 mg) was associated with a fall in V-mca [-11 cm/s,
point estimate of difference versus placebo (95% confidence interval:
-17, -6) cm/s, P<0.001] and a rise in mean arterial pressure [+4 (1, 6
) mmHg, P<0.01] and plasma adrenaline levels [+138 (53, 223) pmol/l, P
<0.01]. After chronic caffeine use, the presser and adrenaline respons
es, but not the drop in V-mca, were significantly attenuated. 3. On ti
lting to 70 degrees the fall in V-mca was greater with placebo than af
ter acute caffeine ingestion [-10 (-14, -15) cm/s, P<0.01], whereas in
crements (above supine values) in heart rate, mean arterial pressure a
nd hormone levels were unchanged by caffeine, In contrast, the adrenal
ine [+126 (29, 282) pmol/l, P<0.01] and noradrenaline [+0.6 (0.1, 0.9)
nmol/l, P<0.05] responses to tilting were augmented after acute caffe
ine ingestion. Chronic caffeine supplementation did not alter the fall
in V-mca associated with tilting, but significantly attenuated the ad
renaline response (P<0.01 compared with the acute study). 4. Acute caf
feine ingestion and orthostasis are both associated with a reduction i
n V-mca and a rise in mean arterial pressure and adrenaline levels. Th
e acute effects of caffeine on mean arterial pressure and adrenaline b
ut not on V-mca are lost with sustained caffeine intake. These results
suggest dissociation between the development of central and periphera
l tolerance after chronic caffeine use.