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Regulation and mechanisms of excitation -contraction coupling in the ventricular myocardium of the surf clam, Spisula solidissima

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  • Biology
  • Animal Physiology
  • Biology
  • Chemistry
  • Medicine
  • Pharmacology


Extensive research has previously examined the extrinsic, neural regulation of the bivalve myocardium, via the action of such neurotransmitters as acetylcholine, serotonin and the tetrapeptide, FRMFamide. This investigation was performed in order to identify any intrinsic, autoregulatory (mechanical or pharmacological) processes, which attenuate or amplifier cardiac activity. For this purpose, the ventricular muscle of the bivalve, Spisula solidissima , was used. The ultrastructure was described in order to explain specific excitation-contraction (EC) coupling mechanisms that were elucidated in this study. ^ Manuscript 1 identified and characterized shortening deactivation in ventricular trabeculae of the surf clam. This is defined as the loss of force when a rapid shortening perturbation is applied during the rising phase of contraction. Deactivation was proportional to the degree of shortening and greatest when this release was applied close to maximal force. It was partially removed with 100μM caffeine and with high extracellular calcium (≥32mM). This work shows in part that muscle length modulates the extent of myofilament calcium ion activation in the bivalve heart, and may explain prior observations of stretch-induced changes in membrane potential. ^ Manuscript 2 presents both histological and pharmacological evidence for the presence of nitric oxide synthase (NOS) and describes the effect of exogenous application of nitric oxide (NO) on ventricular muscle of the surf clam. β-NADPH-diaphorase (NADPH-d) histochemistry was performed on ventricular and atrial whole mounts and detected NOS-specific nerves and diffuse staining throughout the myocardium. The NO donors, S-nitrosoglutathione and S-nitroso-N-penicillamine attenuated muscle contractions with thresholds of 10−7M and 10−5M respectively. With low concentrations of NO donors (<1μM) both methylene blue and 1H-[1,2,4]-oxadiazolo[4,3-a]quinoxalin-1-one (a guanylate cyclase inhibitor), reversed NO-induced inhibition. The above findings suggest NO plays a cardioregulatory role in bivalve cardiac muscle, which is cGMP-dependent. ^ The ultrastructure of the ventricular trabeculae was determined in Manuscript 3. An extensive network of putative sites involved in calcium regulation were located, that included mitochondrial tubules, membrane-bound vesicles and sarcoplasmic reticulum. Physical connections between membrane-associated cisternae and the sarcolemma suggest a possible mechanism for EC-coupling. Myofibrils were arranged similarly to vertebrate smooth muscle. Additional morphological specializations implicated in EC-coupling are discussed. ^

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