Whilst the tissue distribution and range of biological actions reported for apelin suggest a role for the peptide in pressure/volume homeostasis, conflicting reports make the precise role unclear. Furthermore, few integrated studies have been performed and there are no reports of bioactivity of apelin in a large animal model. Accordingly, we have examined the haemodynamic, hormonal and renal effects of apelin in ten normal conscious sheep. Apelin (1 mg i.v. bolus) induced a biphasic haemo-dynamic response characterised by an acute fall in arterial pressure and a rise in heart rate followed immediately by a rise in arterial pressure and a fall in heart rate. The secondary hypertensive phase was associated with a fall in cardiac output (P=0.015) and significant rises in calculated total peripheral resistance (CTPR) (P<0.001) and right atrial pressure (RAP) (P=0.031). Electrocardiogram changes were also observed in four of ten sheep, most notably varying degrees of atrioventricular block. Apelin also induced significant rises in plasma arginine vasopressin (P=0.009), adrenocorticotrophin (P=0.012), aldosterone (P=0.001), cortisol (P=0.014), atrial (P=0.036) and brain (P<0.001) natriuretic peptide, cyclic GMP (P=0.003) and cyclic AMP (P=0.002) levels with no effect on renal indices. In conclusion, high dose administration of apelin to normal conscious sheep induces a significant biphasic response in arterial pressure and heart rate associated with rises in RAP and CTPR and a fall in cardiac output. Apelin also increases circulating levels of a number of vasoactive hormones. Taken together, these results suggest a potential role for apelin in pressure/volume homeostasis.

Emerging evidence suggests that the urocortin (UCN) peptides contribute to pressure and volume regulation with possible involvement in the pathophysiology of cardiovascular disease. We have recently reported that i.v. UCN1 potently inhibits cardiac sympathetic nerve activity (CSNA) in normal sheep. However, little is known about possible interactions between UCN2 and the sympathetic nervous system. Accordingly, we have examined the effects of i.v. UCN2 on CSNA, hemodynamics, and plasma catecholamines in normal conscious sheep. Bolus i.v. administration of UCN2 (25 and 100 μg) resulted in the expected hemodynamic actions including transient falls in arterial pressure (P=0.016) and more sustained rises in heart rate (P<0.001) and cardiac output (P<0.001) and falls in peripheral resistance (P<0.001). CSNA burst frequency showed a biphasic response (P<0.001) with an acute rise followed by a more prolonged fall. CSNA burst area and incidence showed prolonged, dose-dependent falls in response to UCN2 administration (all P<0.001). UCN2 also induced a short-lived rise in plasma norepinephrine levels (P=0.006). The marked rise in heart rate in response to UCN2 is preserved in sheep undergoing pharmacological ganglionic blockade with hexamethonium. In conclusion, this is the first study to report the effects of UCN2 on SNA and indicates potent inhibition of sympathetic traffic to the heart despite a generalized baroreceptor-induced activation of sympathetic activity. These findings suggest an important role for UCN2 in cardiovascular homeostasis and warrant further investigation for potential therapeutic applications in acute myocardial injury and heart disease.

Although C-type natriuretic peptide (CNP) is crucial to post-natal endochondral growth, roles for the hormone in pubertal bone growth and the physiological effects of sex steroid substitution on CNP synthesis are not known. Using a plasma marker of CNP tissue synthesis (amino-terminal proCNP, NTproCNP), we have studied the effect of exogenous oestrogen (E₂) or testosterone (T) on plasma CNP forms and bone alkaline phosphatase (bALP) in pre-pubertal lambs. Responses to E₂ in non-cycling adult ewes were also studied. In 15-week-old intact ewe lambs, E₂ promptly increased plasma NTproCNP and CNP (P<0.001) to peak on day 2, and bALP (P<0.001 peaking on day 7), whereas no significant stimulation in response to T was observed in male lambs. Linear bone growth and live weight were unaffected. In adult anoestrous ewes, basal concentrations of CNP forms and bALP were lower than in ewe lambs, in keeping with skeletal maturity, but adults responded similarly to E₂. Prompt and sustained increases in NTproCNP and CNP, and a later threefold rise in bALP (all P<0.001), were induced by E₂. Possible contributions to these increases in plasma CNP forms by reproductive tissues (a known site of E₂-induced CNP expression) were excluded by showing similar E₂-induced CNP responses in adult ewes after surgical removal of reproductive tissues. These results are the first to show that E₂ stimulates plasma CNP forms and bALP in lambs and adult sheep and raise the possibility that CNP also participates in bone formation in the mature skeleton.

Ghrelin is a 28 amino acid stomach peptide, derived from proghrelin(1–94), that stimulates GH release, appetite and adipose deposition. Recently, a peptide derived from proghrelin(53–75) – also known as obestatin – has been reported to be a physiological antagonist of ghrelin in the rat. Using four specific RIAs, we provide the first characterisation of proghrelin(1–94) peptides in human plasma, their modulation by metabolic manipulation and their distribution in mammalian tissues. ghrelin(1–28) immunoreactivity (IR) in human plasma and rat plasma/stomach consisted of major des-octanoyl and minor octanoylated forms, as determined by HPLC/RIA. Human plasma ghrelin(1–28) IR was significantly suppressed by food intake, oral glucose and 1 mg s.c. glucagon administration. ghrelin(1–28) IR and proghrelin(29–94) IR peptide distributions in the rat indicated that the stomach and gastrointestinal tract contain the highest amounts of the peptides. Human and rat plasma and rat stomach extracts contained a major IR peak of proghrelin(29–94)-like peptide as determined by HPLC/RIA, whereas no obestatin IR was observed. Human plasma proghrelin(29–94)-like IR positively correlated with ghrelin(1–28) IR, was significantly suppressed by food intake and oral glucose and shared with ghrelin(1–28) IR a negative correlation with body mass index. We found no evidence for the existence of obestatin as a unique, endogenous peptide. Rather, our data suggest that circulating and stored peptides derived from the carboxyl terminal of proghrelin (C-ghrelin) are consistent in length with proghrelin(29–94) and respond to metabolic manipulation, at least in man, in similar fashion to ghrelin(1–28).

Sedentary behavior is associated with an attenuated thermogenic response to β-adrenergic receptor (β-AR) stimulation, an important regulator of energy expenditure (EE) in humans. Chronic stimulation of β-ARs, via heightened activity of the sympathoadrenal system, leads to diminished β-AR function. We have investigated the hypothesis that the thermogenic response of sedentary adults to β-AR stimulation will be increased during short-term sympathoadrenal inhibition. Using a randomly ordered, repeated measures study design, resting EE (REE; indirect calorimetry, ventilated hood technique) and the % increase in EE above REE (%ΔEE) during acute i.v. isoproterenol administration (nonselective β-AR agonist; 6, 12, and 24 ng/kg fat-free mass per min) were determined in 16 sedentary adults (nine females and seven males, 25±1 years, body mass index: 26.1±0.9 kg/m², maximal oxygen uptake: 40±2 ml/kg per min (mean±s.e.m.)) in the basal state and on the 6th day of transdermal clonidine administration (centrally acting α2-AR agonist; 0.2 mg/day). Relative to baseline, clonidine inhibited sympathoadrenal activity, as evidenced by decreased plasma norepinephrine concentration (1.04±0.13 vs 0.34±0.03 nmol/l; P<0.001), skeletal muscle sympathetic nerve activity (22.5±3.8 vs 8.5±1.9 bursts/min; P=0.003), and resting heart rate (63±2 vs 49±1 beats/min; P<0.001). Sympathoadrenal inhibition decreased REE (6510±243 vs 5857±218 kJ/day; P<0.001), increased respiratory exchange ratio (0.84±0.01 vs 0.86±0.01; P=0.03), and augmented the thermogenic response to β-AR stimulation (%ΔEE: 11±2, 16±2, and 24±2 vs 14±1, 20±2, and 31±2; P=0.04). These data demonstrate that in sedentary humans, short-term inhibition of sympathoadrenal activity increases the thermogenic response to β-AR stimulation, an important determinant of EE and hence energy balance.