Twenty one of ovariectomized rabbits were divided into 3 groups: the first group for control, the second group for estrogen stimulated group and the third group for progesterone stimulated group.
In each group, after a single, injection of insulin-131I(30-100 ?c.), blood samples were withdrawn at the interval of 10 minutes for 50 minutes and plasma disappearance of injected .insulin-131I with time were observed.
On the basis of radioactivities of insulin-1311 of plasma-time curves, turnover rates of insulin and half time, which is the time required to turnover a half of insulin ;pool, were calulated from each group.
On the other hand, 50 minutes after injection of insulin-1311, animals were sacrificed and various tissues were excised. Weighed tissues were mixed with 10% trichloroacetic acid (TCA) in motor and divided into 2 fractions by centrifugation. Each fraction of tissue homogenate was counted for radioactivity by means of well type scintillation counter.
Insulin-131I spaces in various ´tissues were calculated by obtaining the ratio of radioactivities of TCA precipitable fractions of tissues to that of plasma, while 1311 compound space, which is the distribution spaces of degradated products of injected insulin-1311, were calculated with ratios of radioactivities of TCA soluble fraction of tissues to that of plasma.
Alterations in turnover rates of insulin by ovarian hormons were. analyzed by connecting the distribution spaces of insulin-131I in various tissues of each group.
1) After a single injection of insulin-131I, plasma disappearance of insulin-1311 was decreased exponentially with time showing straight line on the semilogarithmic paper. Therefore, turnover rate can be calculated by obtaining the slope of radioactivities of plasma insulin-1311 vs. time curve and also half time(tj) of insulin turnover by the first order kinetic equation. The turnover rate of insulin was mean of 1.90%/min. in the first group, 2.53%/min. in the second group and 2.18%/min. in the third group. These data showed that turnover rates of insulin were increased by stimulation with ovarian hormones.
2) Insulin-1311 space in the kidney tisseue showed most highest value and averaged about 700%, which value represents about 7 times of concentration of plasma insulin-1311 in the kidney.
In the other tissues insulin-131I space were decreased by in order of spleen, skin, heart, liver, small intestine, uterus, adipose, skeletal muscle and brain tissues. However, there were little differences in insulin -1311 space between control and ovarian hormone stimulated groups.
3) In order to observe the degradation of insulin in various tissues, 1311 compound spaces were determinedby ratio between radioactivities of TCA soluble fractions of tissues and plasma. 1311 compound spaces in kidney, liver, spleen and adipose tissues showed more than 100%, in which tissues radioactivities of TCA soluble fractions were more concentrated than in the plasma. It seems that these tissues would be principal sites of insulin degradation. In the other tissues, 1311 compound spaces were decreased by in order of small intestine, heart, skin, uterus, skeletal muscle and brain tissues. However, there are remarkable ncrease in the 1311 compound spaces of kidney and liver which have insulinase system by stimulation of ovarian hormones and also degradation of insulin-131I was increased in the uterus and skeletal muscle. In the other tissues there were not significant differences in values of 1311 compound spaces between control and ovarian hormone stimulated groups. Degradation rates insulin-131I in the kidney, liver, uterus and skeletal muscle were more prominence in the estrogen stimulated group than in the progesterone stimulated group. From the above data, increase in turnover rates of insulin-1311 by stimulation of ovarian hormones can be explained by acceleration of degradation of insulin at the tissues such as kidney, liver, skeletal muscle and uterus. Possibilities of accelerated degradation of insulin in these tissues were briefly discussed.
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