The inhibitor of phenylethanolamine-N-methyl
transferase (PNMT),
SK&F 64139 (7, 8-dichloro-1, 2, 3, 4-
tetrahydroisoquinoline), when given i.v. (5 mg/kg), did not prevent the decrease of blood pressure induced in conscious spontaneously hypertensive rats (SHR) by
alpha-methyldopa (50 mg/kg i.v.). However, i.c.v. administration of
SK&F 64139 (5 mg/kg) to conscious SHR reduced both the
alpha-methyldopa- and the
clonidine-
induced hypotension.
Bradycardia in response to
clonidine was also prevented by i.c.v.
SK & F 64139. When the
antihypertensive effect of
alpha-methyldopa or
clonidine was fully established, i.c.v. administration of
SK & F 64139 returned blood pressure within a few minutes to the initial value. Similarly, the
bradycardia after
clonidine was promptly reversed by i.c.v.
SK & F 64139. In pithed rats the pressor responses to both
methoxamine and
clonidine were antagonized by
SK & F 64139 suggesting blockade of vascular alpha 1-and alpha 2-adrenoceptors by the PNMT inhibitor. Blockade of central alpha-
adrenoceptors by
SK & F 64139 appears to adequately explain the inhibition of the
antihypertensive effects of
alpha-methyldopa and
clonidine. The present results do not support the claim (Gerkens et al. 1980) that inhibition of the central formation of
alpha-methyladrenaline is the mechanism underlying the antagonism of
alpha-methyldopa-
induced hypotension by
SK & F 64139.