Uptake and retention of moulting hormones by the integument of crayfishes in vitro: I. Influence of temperature, hormone concentration and hormone structure.

Uptake kinetics of [3H]ecdysone into crayfish hypodermis in vitro reveals three different components: a very rapid (within 1 min), highly temperature-dependent (Q10 = 3) and high uptake rate, a second much lower uptake which is only slightly temperature-dependent (Q10 = 1-1.3), and a plateau which is reached after about 20 min at 20 degrees C. Preincubation of tissue with unlabelled hormone reduces the initial uptake rate to a value characteristic for the second, less steep uptake phase. The activation energy for the initial uptake is 45 kJ/mole. At equilibrium the hormone is 4-fold enriched as compared to the medium, the delta G for this enrichment being 3 kJ/mole. The efflux is much lower than the influx and is linear. There is no linear relation between external hormone concentration and initial uptake rate. The initial uptake can be significantly reduced by an excess of diverse ecdysteroids with moulting hormone activity but not by hydrocortisone.