Potentiation of Hypoosmotic Cellular Volume Regulation in the Quahog, Mercenaria mercenaria, by 5-hydroxytryptamine, FMRFamide, and Phorbol Esters

Ventricles isolated from clams (Mercenaria mercenaria) that had been acclimated to 1000 mOsm seawater (SW) release amino acids when incubated in 500 mOsm SW. Taurine, glycine, and alanine account for nearly all of the released amino acids, and total about 37 μmol/g dry tissue weight during a 2-h incubation. The release of amino acids is increased to 69 μmol/g by the addition of 10-6 M 5-hydroxytryptamine (5HT) to the hypoosmotic SW, and to 83 μmol/g by the addition of 10-6 M FMRFamide to the medium. The potentiation of the release by 5HT is blocked by methysergide. The amino acid release is increased by two phorbol esters--phorbol 12,13-diacetate and phorbol 12-acetate, 13-myristate--to 97 and 83 μmol/g, respectively. Forskolin and other cyclic 3',5' adenosine monophosphate agonists have no effect on the release of amino acids in hypoosmotic SW. Phorbol esters, 5HT, and FMRFamide have no effect on the release of amino acids from ventricles incubated in 1000 mOsm SW. Ventricles, first isolated from clams acclimated to 1000 mOsm SW, and then transferred to 500 mOsm SW, increase in wet weight by 20-25%. The increase is maintained for 30 min, and the tissues return their original weight in the ensuing 30 min. The addition of 5HT, FMRFamide, or phorbol esters to the hypoosmotic SW decreases the time necessary for the tissues to return to pre-transfer weights. These results implicate protein kinase C in the responses of bivalve tissues to hypoosmotic media, and suggest that these responses may be modified by neuronal or neurohumoral control.