G. Flik

Kinetics of Cu2+ inhibition of Na+K+-ATPase

The interaction of Cu2+ with enzymatic activity of rabbit kidney Na+/K(+)-ATPase was studied in media with buffered, defined free Cu2+ levels. The IC50-values are 0.1 mumol/l for Na+/K(+)-ATPase and 1 mumol/l for K(+)-pNPPase. Dithiothreitol (DTT) reverses the inhibitory effect of Cu2+ in vitro. Cu2+ exerts non-competitive effects on the enzyme with respect to Na+, K+, ATP or pNPP, but has a mixed-type inhibitory effect with respect to Mg2+. It is concluded that the appreciation of the inhibitory effect of Cu2+ on this enzyme requires carefully composed assay media that include a buffer for Cu2+, and that the IC50-values calculated according to this model indicate that Cu2+ may be more toxic than previously anticipated.

ULTRASTRUCTURE AND DISTRIBUTION DYNAMICS OF CHLORIDE CELLS IN TILAPIA LARVAE IN FRESH WATER AND SEA WATER

Abstract Integumental and branchial chloride cells of tilapia larvae (Oreochromis mossambicus) were studied at the light-microscopical and ultrastructural level. Total numbers and distribution of chloride cells were quantified after immunostaining of cross sections of the entire larvae with an antibody against the α-subunit of Na+/K+-ATPase. The majority (66%) of Na+/K+-ATPase-immunoreactive (ir) cells, i.e. chloride cells, of freshwater tilapia larvae were located extrabranchially up to 48 h after hatching. Five days after hatching, the majority (80%) of chloride cells were found in the buccal cavity. Transfer of 24-h-old larvae to 20% sea water speeded up this process; 24 h after transfer (i.e. 48 h after hatching), the majority (59%) of chloride cells were located in the buccal cavity. The branchial chloride cell population of 24-h- and 120-h-old larvae consisted of immature, mature, apoptotic and necrotic chloride cells. However, relatively more immature chloride cells were observed in freshwater larvae (42–63%) than in (previously studied) freshwater adults (21%), illustrating the developmental state of the gills. After transfer to sea water, the incidence of degenerative chloride cells did not change. Furthermore, the incidence of immature cells had decreased and a new subtype of chloride cells, the ”mitochondria-poor” cells, appeared more frequently. These mitochondria-poor chloride cells were characterised by an abundant tubular system and relatively few mitochondria, which were aligned at the border or concentrated in one part of the cytoplasm. Most of these cells did not contact the water. The function of their enhanced appearance after seawater transfer is unknown.

Activation of a sensorimotor pathway in response to a water temperature drop in a teleost fish

SUMMARY When common carp, Cyprinus carpio L., experience a rapid temperature drop, the cerebral blood volume is strongly reduced to dampen the temperature drop in the brain. Simultaneously, the preoptic area and pituitary gland are activated to launch whole-body adaptive responses. However, the preferred reaction of fish to a temperature change is an escape reaction, which implies activation of a sensorimotor pathway. Here, we used blood oxygenation level-dependent (BOLD)- and cerebral blood volume (CBV)-weighted functional magnetic resonance imaging (fMRI) to identify a sensorimotor pathway, during a 10°C temperature drop in common carp. Transient activation was observed in the region where the sensory root of the trigeminal nerve enters the brain, and in the valvula cerebelli. In both regions, metabolic activity increased (increased deoxyhemoglobin content demonstrated by a decreased BOLD signal) within 30 s after the onset of the temperature drop, peaked after 2-3 min, and then decreased, even though the temperature continued to drop for another 2 min. These brain structures appear to respond to temperature change, rather than to the absolute temperature. Thus, during a temperature drop, the sensorimotor pathway consisting of the trigeminal nerve, the primary sensory trigeminal nucleus, the valvula cerebelli and some motornuclei, is active, in line with perception of temperature change in the buccal cavity, leading to motor activity for escape. This pathway operates in parallel to an acclimation pathway, which involves the preoptic area to pituitary gland pathway.