Characterisation of somatostatin actions on knee joint blood vessels of the rat.

The effects of somatostatin on blood flow, plasma extravasation and knee joint sizes in the rat were investigated. Topical bolus administrations of somatostatin (10 pmol-100 nmol) onto the exposed rat knee joint capsules produced dose-dependent increases in knee joint blood flow with an ED(50) value of 1.7 nmol, and a maximum increase of 109.7%. The peak vasodilator response was observed at 1 min following drug administration, and it subsided at 5 min. Treatment of the rat knee with a somatostatin receptor antagonist cyclo(7-aminoheptanoul-Phe-D-Trp-Lys-Thr[Bzl] (cyclo-somatostatin; 2 x 20 nmol) significantly suppressed the somatostatin-induced vasodilator response, but treatment with the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME; 2 x 50 nmol) or the cyclo-oxygenase inhibitor flurbiprofen (2 x 10 nmol) had no effect. Unilateral intraarticular injections of somatostatin (10 nmol) produced no change on blood flow and sizes of the rat knee joints, but elicited marked ipsilateral Evans blue extravasation. Cyclo-somatostatin at doses of 2 x 20 and 2 x 50 nmol did not affect the plasma extravasation response to somatostatin. The present findings indicate the vasodilator effect of somatostatin is mediated by receptors sensitive to cyclo-somatostatin inhibition, but its plasma extravasation effect might be mediated by somatostatin receptor types that are resistant to inhibition by cyclo-somatostatin. There is no evidence that nitric oxide and prostaglandins are involved in the somatostatin-induced vasodilator response. It is suspected that the vascular effects of somatostatin demonstrated in this study would play a part in the innate response of an inflammatory reaction.