Fish antifreeze proteins block Ca entry into rabbit parietal cells

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Abstract
Many fish and insects have adapted to life at subfreezing temperatures by evolving so-called antifreeze proteins (AFP) that noncolligatively depress the freezing temperatures of aqueous solutions without affecting the melting temperature. AFP have been thought to function solely as antifreezes. Recently, however, we discovered that AFP also protect mammalian cells and organs from damage caused by exposure to hypothermic (above freezing) temperatures. It has been proposed that hypothermic damage is caused by changes in intracellular ionic content due to a reduction of active transport that is required to balance passive ion transport across cell membranes. Given this possibility, we tested whether AFP isolated from the Newfoundland ocean pout might reduce the Ca ion permeability of a mammalian cell, the rabbit gastric parietal cell, which has been particularly well studied in terms of Ca transport and signaling. Digital image processing of the Ca-sensitive fluorescent indicator fura-2 was used to measure intracellular free Ca in these cells. During stimulation with the cholinergic agonist carbachol, AFP inhibited passive Ca entry across the cell membrane without interfering with either the release of Ca from internal stores (indicating that the carbachol receptor and other signaling events were operational) or the normal active rates of Ca efflux from the cell (indicating that Ca pumping was also still intact). These results suggest that, in addition to their actual antifreeze properties, AFP may also help to confer cold tolerance in animals by preventing passive Ca entry into epithelial cells.