β-Adrenergic, Hormonal, and Nervous Influences on Cryoprotectant Synthesis by Liver of the Freeze-Tolerant Wood FrogRana sylvatica

Abstract

The effects of various catecholamines, α- and β-adrenergic agonists and antagonists, hormones, hormone receptor blockers, and other inhibitors, as well as destruction of the nervous system by pithing, were analyzed for their ability to modulate the freezing-induced cryoprotectant output by liver of the wood frogRana sylvatica.Of the numerous treatments tried only intraperitoneal injections of glucagon (a glycogenolytic hormone) and propranolol (a blocker of β-adrenergic receptors), both of which modulate hepatic cyclic AMP levels, altered cryoprotectant output. Glucagon significantly enhanced glucose output into the blood during freezing (by 3.5-fold compared with sham-injected frogs), whereas propranolol significantly reduced the hyperglycemia, liver and blood glucose concentrations being reduced by 55 and 42%, respectively, compared with values for untreated frozen frogs. However, treatment with the α-adrenergic blocker phentolamine did not block freezing-induced cryoprotectant output, indicating that Ca2+-mediated mechanisms are not involved in the hyperglycemic response by liver in these frogs. The mechanism of propranolol action was to suppress the activation of liver glycogen phosphorylase so that the amount of active phosphorylaseain liver of propranolol-treated frogs after freezing exposure was only 5% of the corresponding activity in liver of untreated frozen frogs. The β-adrenergic nature of catecholamine regulation of blood glucose levels was confirmed by the ability of the β-adrenergic agonist isoproterenol to mimic the hyperglycemia induced by epinephrine or norepinephrine injection in unfrozen frogs at 3°C, whereas the α-adrenergic agonist phenylephrine was ineffective. The data indicate that the cryoprotectant output response during freezing ofR. sylvaticais mediated by cAMP-dependent mechanisms within the liver under the control of plasma hyperglycemic hormones.