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Aim: The starting point for this investigation was the observation that, up to day (D) 20 of incubation in the chick embryo is a decreasing heart frequency, and a contradictory increase in noradrenaline (NA) concentration in the blood plasma. The following questions were formulated: (1) What is the reason for this contradictory behaviours of heart frequency and NA at the cellular level? (2) How does temperature influence this process? (3) What is the biological meaning of this process?
Method: White leghorn chicken embryos were used. The control group was incubated continuously at 37.50C. The embryos of the two test groups were incubated up to day 14 at 37.50C. Thereafter, temperature was increased to 38.50C in the warm group and decreased to 35.00C in the cold group. Withdrawal of blood for hormone determination was carried out for all embryos on D 18 and 20 of incubation. In the second study, the same temperature regimes were used. On D 20 embryonic hearts were excised and cardiac muscle cells were isolated. The cells of the control as well as the test groups were stimulated by means of Isoprenalin and the following increase of cAMP was measured.
Results: The NA content in the blood plasma of the embryos showed an age dependent increase between days 18 and 20 (12 to 38 ng/ml). This increase was stronger in the warm and control group than in the cold group. The absolute cAMP content in the embryonic cardiac muscle cells dropped from D 18 to D 20. A stimulation of the cardiac muscle cells by means of Isoprenalin led to a higher percentage of cAMP on D18 (120%) than on D 20 (55%). The cardiac muscle cells of the cold group showed a higher proportional increase in cAMP (60%) after stimulation by means of Isoprenalin than the cardiac muscle cells of the warm group (20%).
Summary: 1. Increasing NA values are combined with decreasing heart frequency and cAMP values on D 20 of incubation. With decreasing heart frequency the duration of diastole is longer compared with the duration of systole. This improves the O2 supply for the heart compared with the time of the restricted oxygen supply during the systole.
2. In the warm group we measured high NA concentrations on D 20, but only a little stimulation of cAMP (20%). Highest cAMP increases (but lowest NA values) are achieved in the cold group on D 20. This effect of sympathetic stimulation in the warm group, compared with the cold group, protects the heart at high temperature (diminishing sensitiveness the NA receptors) and activates the heart at low temperature (increasing sensitiveness the NA receptors).