Avian shell gland contractility: interaction of PGF2 alpha and arginine vasotocin with Ca2+

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Abstract
The effects of prostaglandin F2 alpha (PGF2 alpha) and arginine vasotocin (AVT) on the isometric contractile activity of avian shell gland longitudinal muscle strips were studied in relation to the role of extracellular Ca2+. PGF2 alpha and AVT stimulated contractile tension in a dose-related manner. This was attenuated when either of the Ca2+ channel blockers verapamil or R33956 was added to the muscle chamber baths. Ca2+-free solution containing 1 mM ethyleneglycol-bis (beta-aminoethylether)-N,N'-tetraacetic acid completely prevented contraction in response to increasing doses of PGF2 alpha or AVT. Washing of the strips with Ca2+-free solution eliminated spontaneous contractile activity, but replenishment of CaCl2 to the medium (0.1-5.0 mM) restored it. Addition of PGF2 alpha to the Ca2+-free medium enhanced contractile tension during Ca2+ replenishment, whereas AVT had no effect on tension generation at low extracellular Ca2+ concentration (0.1-0.5 mM) but increased it at higher extracellular Ca2+ concentration (1.0-5.0 mM). PGF2 alpha stimulation was sensitive to extracellular Na+ concentration, whereas AVT-induced activity was not. Potassium depolarization (20 mM K) potentiated PGF2 alpha-stimulated activity, whereas the response to AVT was unaffected. At 127.5 mM K, AVT-stimulated activity was inhibited. PGF2 alpha-enhanced Ca2+-dependent tension generation was right shifted in a dose-related manner by AVT. These results suggest that extracellular Ca2+ is necessary for the full expression of PGF2 alpha- and AVT-stimulated muscle contraction and suggest that each agonist has a different mechanism of action.