SUMMARY

Adenylate cyclase activity of crude plasma membranes from chick kidney was stimulated by low doses of parathyroid hormone (PTH). Sensitivity to PTH was ten to twenty times greater than that of a similar preparation from rat kidney cortex. Synthetic peptides consisting of the NH₂-terminal 34 amino acids of bovine PTH (BPTH) and of human PTH (HPTH) were assayed, as were several analogues of these peptides. Bovine PTH (1–34) and HPTH (1–34) were equivalent in their action on chick kidney but the human peptide had only 20% of the activity of the bovine peptide on rat kidney cortex adenylate cyclase. Bovine proPTH ( −6→ + 34) and (Tyr¹)-BPTH (1–34) had less activity than BPTH (1–34). Bovine PTH (2–34) inhibited the response to BPTH (1–34). Neither salmon calcitonin nor vasopressin stimulated adenylate cyclase activity.

Abstract

Perifused equine anterior pituitary cells were used to investigate the relationships between the secretion of ACTH and substances known to either stimulate (corticotrophin-releasing hormone (CRH), and arginine vasopressin (AVP)) or inhibit (cortisol) ACTH secretion. The experiments were designed to mimic the hormone milieu present in vivo in the horse, with cortisol (0 or 100 nmol/l) and CRH (0 or 0·02 nmol/l) perifused continuously, and pulses of AVP (10 nmol/l) applied for 5 min at 30-min intervals.

In columns perifused with 0·02 nmol CRH/1 there was no significant overall effect of 100 nmol cortisol/l on the ACTH responses to pulses of AVP, although there was a significant interaction between AVP pulse number and cortisol showing that ACTH total area (pmol ACTH proportional to area under response curve) in response to AVP pulses 1 and 2 was significantly (P<0·05) decreased in columns perifused with 100 nmol cortisol/l. However ACTH incremental area (pmol ACTH proportional to the area above the CRH-induced baseline) was not affected by cortisol at any AVP pulse.

This contrasts with the effect of cortisol in columns perifused with 0 nmol CRH/l, where 100 nmol cortisol/l significantly decreased ACTH total area (P=0·0075) and incremental area (P=0·049) at all AVP pulses compared with the responses in columns receiving 0 nmol cortisol/l.

There was a fall off in ACTH responsiveness with time during the experiment which, in the presence of 0·02 nmol CRH/1, was significantly (P<0·001) greater with 0 nmol cortisol/l than with 100 nmol cortisol/l and if 6 (rather than 3) pulses of AVP were given, whereas with 0 nmol CRH/l there was no difference in the fall off with time between columns receiving 0 and 100 nmol cortisol/l.

These results show that the control of ACTH secretion is influenced not only by independent action of secretagogues such as CRH and AVP, or inhibitors such as cortisol, but by a complex interaction of these factors with one another. CRH may have a role in 'protecting' the ACTH response to pulses of AVP in the presence of cortisol. It follows that, in vivo, 'background' CRH could allow an increase in ACTH in response to AVP released by a new stress, despite the presence of elevated cortisol.

Journal of Endocrinology (1996) 148, 475–483

ABSTRACT

A non-surgical, non-stressful technique was used for collection of pituitary venous blood from five conscious horses every minute for two 10-min periods before and during isolation from the herd, which caused a predictable, yet humane and physiological, emotional stress. Pituitary blood was also sampled every 5 min for two approximately 90-min periods before and after isolation, while jugular blood was sampled every 15 min throughout the experiment.

During isolation, all horses became agitated, hyperventilating and sweating. Packed red cell volume increased, as did pituitary venous concentrations of adrenaline (mean ± s.e.m. concentration before isolation, 621·5±112·3 pmol/l; peak during isolation, 2665·4 ± 869·8 pmol/l; P <0·05) and noradrenaline (before, 871·8 ± 111·8 pmol/l; peak, 2726·1 ± 547·4 pmol/l; P<0·02). Concentrations of arginine vasopressin (AVP) were higher in pituitary venous but not in jugular blood during isolation than during the preceding 10-min period (P <0·05). Although AVP secretion increased in all horses, in three of the five it rose dramatically in the first minute of isolation to 25·7 (horse 1), 13·6 (horse 4) and 145·1 (horse 5) times the level in the last sample collected before isolation. Mean pituitary venous concentrations of ACTH and α-MSH increased during isolation in the three horses which had large increases in AVP secretion, but, overall, stress did not significantly affect ACTH or α-MSH secretion. Similarly, mean jugular cortisol levels were not significantly altered by isolation. However, the magnitudes of ACTH, AVP and α-MSH responses to isolation were negatively correlated with the jugular cortisol level before isolation. The changes in pituitary venous concentrations of ACTH and AVP were synchronous under resting conditions, whether samples were collected at intervals of 1 (P <0·01) or 5 (P <0·005) min; however, this synchrony was lost during isolation. The changes in pituitary venous concentrations of ACTH and α-MSH were synchronous both at rest (P <0·025 for 1-min sampling, P <0·01 for 5-min sampling) and during isolation (P<0·01).

We conclude that isolation stress increases AVP secretion and may alter the temporal relationship between pituitary venous concentrations of AVP and ACTH. Furthermore, the magnitude of the responses of AVP, ACTH and α-MSH to isolation is significantly affected by the prevailing cortisol level.

J. Endocr. (1988) 116, 325–334