SUMMARY The β-adrenergic stress response in red blood cells (RBCs) of rainbow trout shows seasonal changes in expression. We have explored the mechanisms underpinning this response by following, over a period of 27 months, changes in β-adrenergic receptor (β-AR) binding characteristics,β-adrenergically stimulated RBC Na + /H + exchanger(βNHE) activity, together with β-AR and βNHE mRNA levels and plasma steroid hormone and lactate levels. These parameters were measured at approximately monthly intervals in a single population of fish held under semi-natural conditions. Membrane-bound, high-affinity β-ARs were present in RBCs at all sampling times, varying from 668±112 receptors cell -1 to 2654±882 receptors cell -1 (mean± s.e.m. ; N =8). βNHE activity, however, was reduced by 57% and 34% in December 1999 and February 2001, respectively,compared with an otherwise sustained influx that averaged 110.4±2.3 mmol l -1 RBCs h -1 ( N =119). Only one reduction coincided with a spawning period but both were preceded by transient increases in circulating testosterone. βNHE activity measured under standard conditions was not correlated with the number or affinity of β-ARs nor with water temperature, but both β-AR numbers and βNHE activity were positively related to their respective mRNA levels ( P =0.005 and 0.038, respectively). Pharmaceutical intervention in the transduction cascade linking the β-AR and βNHE failed to indicate any failure of the transduction elements in RBCs displaying low βNHE activity. Similarly, we failed to demonstrate any link between seasonal cortisol fluctuations and seasonally reduced βNHE activity. However, the βNHE activity of age-separated RBC fractions showed that younger RBCs had a significantly higher βNHE response than older RBCs, consistent with the seasonal reductions in βNHE being linked to turnover of RBCs and erythropoiesis. Testosterone is known to induce erythropoiesis and we conclude that seasonal reductions in βNHE are not caused by changes in β-AR numbers but may be linked to testosterone-induced erythropoiesis.
When held in air for up to 24 h, crayfish accumulated Ca(2+) and Mg(2+) in their haemolymph in direct proportion to raised levels of lactate. K(+) levels were highly variable, with elevated levels associated with morbidity. Lactate accumulation in the haemolymph was reflected in proportional increases in lactate levels in the carapace and muscle. Pieces of carapace incubated in saline containing elevated levels of lactate accumulated lactate to up to half the dissolved concentration. Measured levels in the carapace, relative to its water content, implied that lactate accumulated in the carapace in a combined form, possibly complexed to calcium. The exoskeleton seems to provide a reserve of buffering capacity and a sink for lactate during anaerobic metabolism. A similar mechanism has been identified in pond turtles.