Long-term Xenopus laevis tadpole -heart-organ-culture: Physiological changes in cholinergic and adrenergic sensitivities of tadpole heart with thyroxine-treatment

The present study clarified changes in physiological sensitivities of cultured Nieuwkoop and Faber stage 57 Xenopus laevis tadpole-organ-heart exposed to thyroxine (T₄) using acetylcholine (ACh), norepinephrine (NE) and atropine. For preliminary life span and the chemical tests, 60% minimum essential medium (MEM), two types of modified Hank's balanced salt-solution-culture-media (MHBSS-CM) I and II containing relatively lower concentrations of amino acids and collagen were prepared. In preliminary lifespan-test of cultured tadpole hearts, the hearts maintained in 60% MEM was 50 days on average, whereas that of the tadpole-hearts in MHBSS-CMs was extended by 109 days on average, showing superior effectiveness of MHBSS-CMs. 4 min-stimulation by 5 × 10⁻⁹ M T₄ tended to increase the tadpole heartbeat. 10⁻⁹ M ACh decreased the tadpole heartbeat. Frog-heart at 2–4 weeks after metamorphosis completion and tadpole heart treated with 5 × 10⁻¹⁰ M T₄ for 45 h also responded to 10⁻⁹ M ACh, and low-resting hearts were restored to the control level with the competitive muscarinic antagonist 10⁻⁸ M atropine, whereas excessive exposure of 10⁻⁵ M atropine to T₄-treated tadpole heart did not increase heartbeat in spite of the increased frog heartbeat over the control. 10⁻¹⁴ —10⁻¹² M NE increase the tadpole heartbeat in a concentration-dependent manner, however, 10⁻¹² M NE did not act to stimulate adrenergic receptors on both T₄-treated tadpole- and the frog-hearts. These results suggest that T₄ induces the desensitization of atropine-sensitive muscarinic and adrenergic receptors in organ-cultured tadpole-heart.