Osmotic versus adrenergic control of ion transport by ionocytes of Fundulus heteroclitus in the cold

In eurythermic vertebrates, acclimation to the cold may produce changes in physiological control systems. We hypothesize that relatively direct osmosensitive control will operate better than adrenergic receptor mediated control of ion transport in cold vs. warm conditions. Fish were acclimated to full strength seawater (SW) at 21 °C and 5 °C for four weeks, gill samples and blood were taken and opercular epithelia mounted in Ussing style chambers. Short-circuit current (Isc) at 21 °C and 5 °C (measured at acclimation temperature), was significantly inhibited by the α2-adrenergic agonist clonidine but the ED50 dose was significantly higher in cold conditions (93.8 ± 16.4 nM) than in warm epithelia (47.8 ± 8.1 nM) and the maximum inhibition was significantly lower in cold (− 66.1 ± 2.2%) vs. warm conditions (− 85.6 ± 1.3%), indicating lower sensitivity in the cold. β-Adrenergic responses were unchanged. Hypotonic inhibition of Isc, was higher in warm acclimated (− 95%), compared to cold acclimated fish (− 75%), while hypertonic stimulations were the same, indicating equal responsiveness to hyperosmotic stimuli. Plasma osmolality was significantly elevated in cold acclimated fish and, by TEM, gill ionocytes from cold acclimated fish had significantly shorter mitochondria. These data are consistent with a shift in these eurythermic animals from complex adrenergic control to relatively simple biomechanical osmotic control of ion secretion in the cold.