Nitrergic modulation of molluscan hearts

White, Anthony Ronald (1999) Nitrergic modulation of molluscan hearts. University of Southampton, Doctoral Thesis.

Abstract

Hearts were removed from Achatina fulica, the giant African land snail, and Helix aspersa, the common British garden snail, suspended in an organ bath continually perfused with saline.

Four nitric oxide (NO) donors, sodium nitroprusside (SNP), acidified sodium nitrite (NaNO₂), S-nitroso-N-acetyl-D, L-penicillamine (SNAP) and S-nitroso-L-glutathione (SNOG) were examined for their action on the hearts. All the donors gave a concentration-dependent inhibition of activity of both hearts except for SNP which, on Helix heart only, gave an initial excitatory response followed by inhibition. A rank order of potency, was established for both hearts.

On Achatina heart, methylene blue was found to reversibly reduce the inhibitory effect of SNAP, SNP and NaNO₂. The phosphodiesterase inhibitor, 3-isobutyl-methylxanthine (IBMX), reversibly enhanced the inhibitory effect of SNP and SNAP. The soluble cyclic-guanosine monophosphate analogue (cGMP), 8-bromo cyclic-guanosine monophosphate (8-Br-cGMP), potentiated the response to both SNAP and SNP, this effect appearing to be non-reversible. The selective inhibitor of NO-activated guanylyl cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), reversibly inhibited the response of the heart to SNOG and acetylcholine (ACh), Methylene blue and ODQ directly excited the heart. cGMP and IBMX directly inhibited the heart.

NADPH-diaphorase (NADPH-d) staining of Achatina heart resulted in consistent staining of the pericardium with less intense staining of the inner myocardium. On Helix heart, methylene blue was also found to reversibly reduce the inhibitory effect of SNAP, SNP and NaNO₂. Potassium (K⁺) free ringer depressed the response to SNAP. IBMX reversibly potentiated the effect of NaNO₂ and SNAP. 8-Br-cGMP potentiated the response to SNAP. A significant increase in intracellular cGMP levels was demonstrated after Helix hearts were treated with SNAP and ACh. These data suggest that a nitrergic system maybe involved in the regulation of cardiac activity of gastropod molluscs.

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