conformation of the membrane, alters membrane permeability more or less 

 selectively. Species variation would seem to rest: 1) with quantitative 

 differences in concentration of neurohumors , enzyme systems, and receptor 

 sites; and 2) with variability of receptors, and of the particular 

 association they make from among available ionic channels. - J.L.M. 



705 



Greenberg, Michael J. 1970. 



A comparison of acetylcholine structure-activity relations on the hearts of 

 bivalve molluscs. Comp. Biochem. Physiol. 33(2): 259-294. 



The experiments described in this paper deal with Mytilus planulatus , M. 

 edulis, Spisula solidissima and Tresus capax. Mereenaria mereenaria is 

 referred to frequently, but only with reference to published work abstracted 

 elsewhere in this bibliography, especially what are described as a series of 

 classical investigations on the isolated "standard" heart of M. mereenaria 

 by Welsh and Taub (1948-1953). - J.L.M. and M.W.S. 



706 



Greenberg, Michael J., and Thomas C. Jegla. 1962. 



The pharmacology of the rectum of Mereenaria (Venus) mereenaria. Amer. Zool. 

 2 (3) : 412 (abstract) . 



The rectum is located within the ventricle, to which it is attached at either 

 end. Longitudinal muscle occurs in this segment. An isolated rectum was 

 prepared by stripping all ventricular muscle and suspending the rectum from a 

 500 mg isotonic lever in a seawater bath at 15°C. 5-Hydroxytryptamine (5HT) 

 excites the rectum. Threshold is about 3xl0~8 M. Low doses (10~6 m) often 

 induce rhythmic activity. Higher concentrations inhibit beat and greatly 

 increase tone. Benzoquinonium chloride (5xl0~^ M) antagonizes this positive 

 intropic effect and induces a vigorous beat. Curare (10~5 g/ml) also 

 inhibits the tone increase with 5HT. Atropine (5xl0~5 m) either has no 

 effect or potentiates the positive tonotropic effect. Atropine has no effect 

 on rhythmicity induced by 5HT. With increasing concentration acetylcholine 

 (ACh) first depresses, then increases tone of a nonbeating rectum. Depressor 

 threshold is about 10 -8 M; max depression is achieved between 10 -7 and 10~6 

 M. Excitor threshold is 3xl0 -6 to 10 -5 M. Guts induced to beat by 5HT 

 respond to ACh as do hearts of most bivalves (but not Mereenaria) . Tone, 

 amplitude and frequency of beat are decreased by low concentrations, while 

 higher doses produce large tone increase and obliterate beat. All actions of 

 ACh are at least partially blocked by benzoquinonium chloride. It is suggested 

 that 5HT not only acts directly on gut musculature, but also excites neurons 

 which release ACh. Beat at low 5HT concentrations is explained by a 

 requirement of a precise 5HT:ACh ratio for beat. - J.L.M. 



707 



Greenberg, M. J., and T. C. Jegla. 1962. 



Histology of the bivalve rectum. Am. Zool. 2(3): 526 (abstract). 



Mereenaria mereenaria is not mentioned specifically but it probably was one 

 of many species examined. The wall of the rectum has an outer layer of 

 connective tissue and an inner layer of secretory simple ciliated columnar 

 epithelium lining the lumen and resting on a basement membrane. The 

 connective tissue layer has 3 conspicuous components: 1) longitudinal 

 collagen fibers, 1-3 u diam, from the periphery to the center of the layer; 

 2) longitudinal unstriated muscle cells usually occupy the middle region of 

 the wall - these cells (250x4 u) have a central nucleus and peripherally 

 distributed myofibrils - muscle inserts on the collagen fibers, which pleat 

 when the rectum shortens; 3) a layer of elastic and reticular fibers between 

 musculature and basement membrane. The wall: lumen ratio varies from 0.3 to 

 0.025 in species examined. Extent of folding of inner rectal wall is related 

 to size of animal. Small animals like Donax have smooth-bore rectums , but in 

 larger species the wall is highly sculptured. - J.L.M. 



198 



