En. Su et al., EFFECTS OF EXTRACELLULAR PH ON AGONIST-INDUCED VASCULAR TONE OF THE CAT OPHTHALMOCILIARY ARTERY, Investigative ophthalmology & visual science, 35(3), 1994, pp. 998-1007
Purpose. To test whether changes in extracellular pH (pHe) in an in vi
tro preparation of the cat ophthalmociliary artery affect passive tone
and agonist responses and whether the endothelial cells are mediators
of any pH-induced effect. This will determine the ability of the opht
halmociliary artery to influence retinal and choroidal blood flow in r
esponse to metabolic stimuli. Methods. The isometric tension generated
by isolated ring segments preactivated by prostaglandin F-2 alpha (PG
F(2 alpha)), noradrenaline (NA), or 40 mM K+ was measured as the pHe o
f the bathing solution was changed stepwise from 6.0 to 8.0 by adjusti
ng the bathing bicarbonate concentration in preparations with and with
out functioning endothelial cells. Results. PGF(2 alpha) produces a co
ncentration-dependent contraction that is insensitive to an alkaline s
hift from control pHe (7.4) in the bathing medium. For acidic shifts t
o pHe 7.0, there is no significant change in the magnitude of the PGF(
2 alpha) contraction, whereas at pHe 6.0, the PGF(2 alpha) contraction
is reduced to 23 +/- 4% (n = 23) of its value at pHe 7.4. Threshold r
esponse concentration remains unaffected. Deliberate damage to the end
othelial cells does not significantly affect the magnitude of the 10(-
5) M PGF(2 alpha) response at pHe 7.4 nor the effect of acidic pH on t
his response. The 10(-5) M NA response is reduced in a graded fashion
to acidic shifts to pH 7.0 and 6.0 (40 +/- 4% [n = 23]) and also to al
kaline shift to pH 8.0 (22 +/- 5% [n = 23]) when compared to the induc
ed tension at pHe 7.4. For the acidic shift only, endothelial cell dam
age causes a further significant reduction in the NA response to 20 +/
- 3% (n = 5). For vessels contracted with K+-Krebs solution, there is
a small but significant reduction in response at pHe 6.0 to 84 +/- 6%
(n = 25), whereas for pHe 8.0 there is a much larger reduction to 45 /-: 5% (n = 24). All pile-induced relaxations of K+ are endothelium in
dependent. Passive tension is unaffected by all pHe manipulations. Con
clusions. Vessel responses to vasoactive agents are selectively mediat
ed by pHe changes. Major acidic shifts cause reduced responses (relaxa
tion) to NA, PGF(2 alpha), or K+, whereas only vessels preactivated wi
th NA and K+ relax to alkaline shifts. This implies that NA or K+ indu
ced vascular responses are maximal close to neutral pHe with major shi
fts from neutrality in either the acidic or alkaline direction causing
a reduced response. These results imply that the ophthalmociliary art
ery probably does not play a major role in controlling ocular blood ho
w in response to pHe changes within the normal metabolic range, but it
may became important in ischemic conditions.