AJP - Endocrinology and Metabolism, Vol 247, Issue 3 389-E397, Copyright © 1984 by American Physiological Society
Effects of adrenalectomy on activation of glycogen phosphorylase in rat myocardium
J. B. Reese, S. E. Mayer and L. L. Brunton
Adrenalectomy causes a depressed glycogenolytic response to catecholamines
in myocardium. Total phosphorylase activity (a + b) is 20% lower in
isolated, perfused hearts from adrenalectomized (ADX) rats compared with
hearts from sham-operated (sham) rats even though the basal activity ratios
(-AMP/+AMP) do not differ. In response to epinephrine (50 nM), the sham
group has a higher activity ratio than the ADX group (0.23 vs. 0.16); the
difference in specific activities of phosphorylase a in the two groups is
even greater, 87 versus 49 U/mg protein. The glycogen content of the heart
is 30% lower in the ADX group. Adrenalectomy does not alter the
accumulation of cAMP and activation of cAMP-dependent protein kinase caused
by epinephrine. Although rat heart contains a heat-stable phosphatase
inhibitor, the activity of this inhibitor, as judged by phosphorylase
phosphatase activity, is not altered by epinephrine stimulation or by
adrenalectomy. Epinephrine perfusion increases the activity ratios (pH
6.8:8.2) of phosphorylase kinase equally in sham and ADX hearts; however,
the specific activities of phosphorylase kinase (basal and
hormone-stimulated) at either pH are lower after adrenalectomy. The
sensitivity of phosphorylase kinase activity to stimulation by calcium is
the same in the sham and ADX groups. A radioimmunoassay for phosphorylase
kinase detects 10% less of this enzyme in hearts from adrenalectomized
animals. Specific activities at pH 6.8 and 8.2 based on the quantity of
phosphorylase kinase detected by radioimmunoassay suggest a lower
phosphorylation state in the ADX group. Decreases in quantities of
phosphorylase and phosphorylase kinase and enzyme dissociation due to
glycogen depletion could all contribute to a depressed glycogenolytic
response in the ADX group.
