KINETICS OF RAPID ⁴⁵Ca UPTAKE INTO CHICK SKELETON IN VIVO: EFFECTS OF MICROWAVE FIXATION

¹ Department of Child Health, St Mary's Hospital, Manchester M13 OJH
² School of Pharmacy and Biomedical Sciences, Portsmouth Polytechnic, St Michael's Building, White Swan Road, Portsmouth PO1 2DT, Hants
³ To whom correspondence should be addressed

We have investigated uptake kinetics for ⁴⁵Ca labels acutely administered as single intravenous injections into twelve-day-old chicks in vivo. Effects of microwave fixation on this process have aJlso been studied. Rapid uptake of ⁴⁵Ca was demonstrated into the skeleton with approximately 40% of injected label being found in the skeleton within the first 15 min. By 45 min tissue ⁴⁵Ca levels had stabilized and showed little change during the following 90 min. Soft tissue isotope levels declined during the period from 3 to 135 min following injection. These data were obtained from animals in which tissue isotope levels were stabilized by fixation with microwaves at the point of killing by cervical dislocation. In animals where the microwave fixation step was omitted and tissues were dissected for counting at least 1 h after killing, ⁴⁵Ca levels in both skeletal and soft tissues were 15-20 % higher than in those from microwaved groups during the period 3-15 min after injection. In a separate experiment, groups of chicks killed by cervical dislocation 3 min after isotope injection were fixed with microwaves at time intervals ranging from 0 to 45 min post-mortem. Isotope levels in femur increased with time and were significantly higher in groups in which fixation was carried out 12 and 45 min after death when compared with those fixed at the point of death. Data in calvarium reflected those in femur but were not statistically significant at the time intervals tested. Estimates of unidirectional plasma-bone Ca²⁺ influx (J_Pb ⁴⁰Ca) into chick bone indicate high values for femur, 1·64, µmol min-1 (g wet weight)^-1 and calvarium, 1·26 µmol min^-1 (g wet weight)^-1, respectively. This suggests that Ca²⁺ is cleared very rapidly between blood plasma and exchangeable pools in the skeleton of the chick. Estimated J_Pb ⁴⁰Ca values decline appreciably over short (15 min) time intervals, suggesting that significant backflux of the ⁴⁵Ca marker occurs. These data collectively indicate that uptake of ⁴⁵Ca into skeletal tissues occurs rapidly following its injection into chicks. ⁴⁵Ca uptake continues for several minutes in the immediately post-mortem animal and this process is reduced by microwave fixation. It is important to bear these factors in mind, when considering the effects of hormones and drugs on the uptake of bone-seeking radionuclides.