NERVOUS CONTROL OF THE SPLEEN IN THE RED-BLOODED ANTARCTIC FISH, PAGOTHENIA-BORCHGREVINKI

The mechanisms of splenic control in the Antarctic fish, Pagothenia bo rchgrevinki, were investigated using isolated spleen and mesenteric ar tery strips in vitro and perfused spleen preparations in situ. Splenos omatic index (SSI) [100 x (spleen wt/body wt)] and hematocrit were det ermined in animals treated with atropine and phentolamine. Atropine in jection increased the SSI from 0.60 +/- 0.06 to 0.89 +/- 0.04, whereas phentolamine decreased SSI to 0.45 +/- 0.03. In atropine-injected fis h, hematocrit was 18.6 +/- 1.4 before and 6.6 +/- 0.8% 3 h after injec tion. Electrical stimulation of the splenic nerves produced biphasic f low responses. In 11 of 12 tested preparations, atropine (3 x 10(-7) t o 10(-6) NI) abolished the response, suggesting a major cholinergic co mponent in the splenic innervation. Isolated spleen strip prepara tion s contracted in response to carbachol, a response that was antagonized by atropine. The response to acetylcholine was markedly enhanced by t he specific cholinesterase inhibitor BW-284c51. Catecholamine effects were somewhat irregular, and maximal contraction force with epinephrin e and norepinephrine was 41 and 56%, respectively, of the carbachol re sponse. The results suggest a mainly, if not solely, cholinergic auton omic control of the borch spleen, and a major function of the choliner gic innervation in the control of hematocrit in this species.