SUMMARY1. Single muscle fibres from the barnacle Balanus nubilus have been studied to provide information about the mode of action of aldosterone on Na transport in a symmetric cell.2. Neither internal nor external application of aldosterone modified the course of the Na efflux. However, fibres pre-exposed overnight to a dose of aldosterone showed a biphasic response to external aldosterone. The first phase was prompt in onset and consisted of a falling rate constant for Na efflux becoming a constant. This has been interpreted as indicating that aldosterone acts by releasing sequestered or bound Na. The second phase was delayed in onset with the average latent period being 68 min. It consisted of a step-up in the rate of Na efflux, followed by a transitory rise in the efflux rate constant. This second phase was dose-dependent, the minimum concentration being 10-9 M. 3. Internal application of aldosterone in a concentration as low as 10-1o M promptly stopped the rate constant for Na efflux from further falling but failed to cause delayed stimulation.4. The response of the Na efflux into Li-ASW following external application of aldosterone was bi-phasic. The magnitude of the delayed stimulation was comparable to that found in controls.5. No correlation between latent period or size of the internal Na bound fraction, and the magnitude ofthe delayed stimulation could be established.6. Internal application of actinomycin-D, oc-amanitin or cordycepin shortly before application of external aldosterone caused complete abolition of the delayed stimulation.7. Overnight pre-exposure of the barnacle to actinomycin-D caused complete abolition of the falling rate constant for Na efflux, as well as the delayed stimulation caused by external aldosterone.8. Internal applicationof spironolactone SC-14266 shortlybefore external E. E. BITTAR AND R. B. TALLITSCH application of aldosterone caused complete abolition of the biphasic response to the steroid.9. It is concluded that (i) barnacle fibres can be made sensitive to aldosterone, (ii) the biphasic effects on the Na efflux depend on prior induction of RNA, (iii) the first effect caused by internal or external aldosterone involves mobilization of sequestered Na, (iv) the results obtained with spironolactone are in keeping with the current view that aldosterone interacts with a receptor system before causing de-repression.10. The implications of the finding that aldosterone releases sequestered Na are briefly touched upon.

SUMMARY1. A study has been made of the nature of the delayed stimulation caused by external aldosterone in barnacle fibres pre-exposed to aldosterone.2. (i) Microinjection of 0*5 M-ATPMg2-caused only a small but prompt rise in the Na efflux.(ii) Microinjection of 0-5 M-ATPMg2-followed by external application of 10-5 M aldosterone greatly augmented the magnitude of the delayed stimulation. The response was dose-dependent, as well as dependent on the concentration of external K+ and H+, but not Na+, Ca2+ or Mg2+.(iii) External application of 10-5 M aldosterone for 30 min followed by its withdrawal from the bathing medium failed to bring about delayed stimulation. By contrast, fibres into which ATP had been injected showed delayed stimulation under these conditions.3. Microinjection of actinomycin-D or spironolactone SC-14266 into fibres into which ATP had been injected followed by external application of aldosterone resulted in complete abolition of the delayed stimulation.4. Delayed stimulation was reduced whether ATP had been injected or not by prior external application of 10-4 M ouabain or internal application of 8 x 10-2 M ethacrynic acid. It was completely abolished by prior application of ouabain externally and ethacrynic acid internally, or only 104 M diphenylhydantoin externally.5. (i) Microinjection of atractyloside or bongkrekic acid caused a substantial fall in the resting Na efflux. Bongkrekic acid proved more powerful than atractyloside. Microinjection of 0-05 M-ATPMg2-into fibres poisoned with 2-0 x 10-2 M bongkrekic acid completely restored the Na efflux.E. E. BITTAR AND R. B. TALLITSCH Ouabain caused no reduction in the bongkrekic acid-insensitive Na efflux, whereas bongkrekic acid caused a reduction in the ouabaininsensitive Na efflux.(ii) Microinjection of atractyloside or bongkrekic acid prior to external application of aldosterone resulted in complete abolition of the delayed response.6. (i) Microinjection of 10-2 M pyruvate caused a brief small rise in the rate constant for Na efflux and when followed by external application of aldosterone there was an enhanced delayed response.(ii) Microinjection of 10-2 M oxythiamine caused a gradual fall in the resting Na efflux and abolished the delayed stimulation caused by external aldosterone. Fibres into which pyruvate had been injected when injected with oxythiamine behaved towards external aldosterone in the same was as control fibres injected with oxythiamine.7. Microinjection of phosphatidyl-L-serine (20 mg/ml.) before external application of aldosterone failed to result in an enhanced delayed response.8. It is concluded that the delayed response in unenriched and ATPenriched fibres depends on stimulation by aldosterone of new RNA and protein synthesis, and that this induction mechanism leads to an increased activity of two membrane enzyme systems, one ouabain-sensitive and the other ethacrynic acid-sensitive. The argument that the ethacrynic acid-sensitive component of the resting Na efflux and of the aldosteronestimulated Na efflux are closely c...

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Graduate students in physiology programs today are faced with a job market that is significantly different from that which many of us faced 25, 10, or even 5 years ago. As a result, physiology educators need to change the content and style of their programs to prepare graduates for their futures. Programs must continue to prepare graduates for research postdoctoral positions, but they also should prepare them for employment at smaller institutions that have higher teaching demands and that encourage, but do not require, research. Changes are suggested in the way PhD candidates are taught and in the way professors mentor students seeking employment at smaller institutions.