Abstract
The open-chest rat preparation was used to investigate the interaction between corticosterone and thyroxine in regulating myo-cardial phosphorylase activity; measurements were also made of heart rate and mean arterial blood pressure. Corticosterone, but not thyroxine, caused an increase in total heart phos-phorylase which was due to a specific increase in the enzyme rather than a general increase in cellular protein. Both corticosterone and thyroxine increased cardiac phosphorylase a activity n i normal rats. The enzymatic and positive chronotropic effects of thyroxine were markedly diminished in adrenalectomized rats; simultaneous administration of corticosterone with thyroxine restored the responses to normal. In thy-roidectomized animals, corticosterone produced less stimulation of heart phosphorylase a than in control rats. When both thyroxine and corti-costerone were given to thyroidectomized animals, the level of phosphorylase a approximated that seen when these hormones were injected into normal ra...
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