The antagonistic effects of 5‐hydroxytryptamine and methylxanthine on the gill cilia of mytilus edulis

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Abstract
The laterofrontal (LF) cirri on isolated gill filaments of Mytilus edulis, prepared in natural seawater, are active and initially beat with an average frequency of about 8 Hz (with a range of 6–14 Hz). However, the lateral (L) cilia on these filaments are arrested in a position at the end of their recovery stroke. Perfusion of the filament with artificial seawater (ASW), with or without 1% ethanol, has little or no biological effect on the activity of the LF cirri, although a transitory decrease in frequency often accompanies the perfusion process. The L cilia remain arrested during perfusion with ASW. The exposure of the gill to low levels of 5-hydroxytryptamine (5HT) (10−8 < 5HT < 10−7 M) has no effect on the activity of the LF cirri but stimulates the L cilia to beat. Exposure to higher concentrations of 5HT (> 10−7 M) elevates the beat frequency of the L cilia and simultaneously inhibits the activity of the LF cirri, leading to their arrest in a position at the end of the effective stroke. This arrest of the LF cirri occurs as the L cilia attain a 5HT-induced beat frequency between 12 to 14 Hz. The influence of 5HT on the L cilia and the LF cirri can be reversibly mimicked or enhanced by the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX). A concentration of 0.5 mM IBMX mimics low 5HT concentrations (about 10−7 M) by stimulating the L cilia to beat without affecting the beat frequency of the LF cirri. A combimation of 10−7 M 5HT and 0.5 mM IBMX in ASW mimics high (> 10−6 M) 5HT concentrations by arresting the LF cirri and increasing the beat frequency of the L cilia. Under these conditions, the threshold of the LF cirri arrest response is again found to occur as the L cilia attain a beat frequency of 12 – 14Hz. These results suggest that the mechanisms of LF cirri arrest and L cilia activation are mediated by 5HT -induced changes in intracellular cyclic AMP levels.