CARDIOVASCULAR CONTROL IN ANTARCTIC NOTOTHENIOID FISHES

The temperatures in the polar oceans are not only low, but also relati vely stable. Blood becomes more viscous at cold temperatures and it is assumed that this increase of viscosity is responsible for a number o f adaptations of the cardiovascular system. The Antarctic Nototheniids show large changes in haematocrit compared to other fishes, and this phenomenon may be related to the high viscosity of their blood at low temperatures. Reduction of the haematocrit will reduce the viscosity, and thus diminish cardiac work. Indeed, one group (the ''icefish,'' Ch annichthydae) has disposed of erythrocytes altogether. The cholinergic tonus on the heart is remarkably high under ''resting'' conditions-up to 80% in the bottom-dwelling Trematomus bernacchii- and changes in c ardiac performance appear to depend chiefly on modulation of this tonu s, rather than activity in excitatory (adrenergic) fibres. Sequesterin g of erythrocytes by the spleen is a major factor in the reduction of haematocrit, and cholinergic autonomic nerves control release of these cells to increase haematocrit during periods of demand. Thus, the stu dies of the autonomic control of the heart and spleen of the Antarctic fish show that these are unusual among fishes in that both organs app ear more or less solely cholinergically controlled. (C) 1997 Elsevier Science Inc.