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Preparations of capsular rat adrenal cells consisting mainly of zona glomerulosa with less than 5% zona fasciculata contamination are described. The responses of the aldosterone and corticosterone outputs of these preparations to various stimuli were of four types. (1) Variations in K⁺ concentration gave a maximum aldosterone response at 5·9–8·4 mM-K⁺, about sixfold greater than the control output at 3·6 mmol/l. At higher K⁺ concentrations, such as 13 mmol/l, the response decreased. (2) Serotonin (at a concentration of about 10^–4 mol/l) gave only a slightly lower maximal aldosterone response than did K⁺ but this did not decrease significantly at higher concentrations. Serotonin gave significant steroidogenic response at 10^–8 mol/l. (3) [Asp¹,Val⁵]-Angiotensin II (10^–10 mol/l) with 3·6 mM-K⁺ gave a significant response and a constant maximal response at 2·5 x 10^–8 mol/l. This maximum response was about half that found for both aldosterone and corticosterone when stimulated maximally by K⁺ or serotonin: [des-Asp¹,Ile⁵]- and [des-Asp¹,Val⁵]-angiotensin II (angiotensin III) gave similar response characteristics but had a lower potency in this cell preparation. The initial maximum response could be further increased at a higher concentration (from 2·5 x 10^–5 mol/l) of a preparation of [Asn¹,Val⁵]-amide angiotensin II (Hypertensin-Ciba) and might eventually be greater than with K⁺. This additional response was, to a major extent, due to stimulation of the contaminating zona fasciculata cells and was not seen with high concentrations of the free acid, angiotensin II. It was also not seen in two experiments with pure [Asn¹]-amide angiotensin II and therefore it could have been due to some impurity in Hypertensin-Ciba. (4) Adrenocorticotrophin (Synacthen) at 3 x 10^–11 mol/l gave a significant steroidogenic response. Higher concentrations (3 x 10^–10 to 7·5 x 10^–9 mol/l) gave no constant maximum but the response could be much greater than for other stimuli such as K⁺, serotonin and [Asp¹]-angiotensin II. This additional response was again due to steroid precursors, e.g. deoxycorticosterone and corticosterone from contaminating zona fasciculata cells. Similar results were obtained with ACTH (ACTHAR) in three experiments. Threshold sensitivity (a significant increase in steroidogenesis) for ACTH (Synacthen) was, in two experiments, greater for zona fasciculata-reticularis cells (3 x 10^–12 mol/l) than for zona glomerulosa cells (3 x 10^–11 mol/l).

The data show that aldosterone output was approximately a function of the square of the corresponding corticosterone value. Specific effects on this pathway can be shown by values of aldosterone/corticosterone² greater than one. Of all stimuli used, only K⁺ concentrations of 5·3, 5·9 and 13 mmol/l gave such effects. However, because of several considerations, only positive results with other stimuli may be meaningful. Calculation of this parameter might be useful as a screening test in bioassays for substances with aldosterone-stimulating activity.

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