Cold acclimation allows regulation of chloride secretion in a eurythermic teleost fish Fundulus heteroclitus

Fundulus heteroclitus (mummichog or common killifish) is an ideal model for ion transport regulation in chloride cells of the opercular epithelium (OE) and the response to thermal challenge. Mummichogs were acclimated to warm (20 °C) and cold (5 °C) seawater and opercular epithelia dissected and mounted in isolated Ussing-style epithelia chambers. The α2 adrenergic agonist clonidine inhibited the Cl(-) secretion (measured as short-circuit current, Isc), while the β-adrenergic agonist isoproterenol and 1.0mM dibutyryl cyclic adenosine monophosphate (db-cAMP) plus 0.1mM isobutyl methylxanthine (IBMX) stimulated Isc in OE from warm and cold acclimated fish, measured at 20 °C. In contrast, rapid cooling partially inhibited Isc, but totally blocked the inhibition by clonidine and stimulation by isoproterenol and db-cAMP+IBMX in OE from warm-acclimated fish, while OE from cold-acclimated animals responded normally at 5 °C. Warming epithelia from 5 °C to 20 °C restored Isc and stimulation by db-cAMP+IBMX markedly increased Isc to levels similar to warm acclimated epithelia, while isoproterenol was much less effective. The isoproterenol insensitivity suggests a downregulation of β-adrenergic receptors in the cold. We infer from present results and previous work (Buhariwalla et al. 2012) that cold shock of plasma membranes induces a phase shift from liquid to gel state that impaired plasma membrane protein mobility of necessary hormone regulatory functions, while cold acclimation preserved ion transport regulation via homeoviscous adaptation of plasma membrane lipids.