Anterior pituitary function during critical illness and dopamine treatment

The neuroendocrine response to stress is a dynamic process

The initial neuroendocrine response to critical illness illness consists primarily of activated anterior pituitary function, the peripheral anabolic pathways being inactivated. This response presumably provides metabolic substrates, establishes the host's defences and is thus considered to be adaptive and beneficial. It was previously assumed that the acute stress response persisted throughout the course of critical illness, but this assumption has now been disproved. Indeed, a uniformly reduced pulsatile secretion of growth hormone, thyroid-stimulating hormone, prolactin and luteinizing hormone has been observed in protracted critical illness, impairing the function of target organs. A reduced availability of thyrotropin-releasing hormone, gonadotropin-releasing hormone, the endogenous ligand of the growth hormone-releasing peptide receptor (possibly ghrelin) and, in very long-stay critically ill men, also growth hormone-releasing hormone seems to be involved. The pulsatile secretion of growth hormone, thyroid-stimulating hormone, prolactin and luteinizing hormone can be re-established by relevant combinations of releasing factors, which also substantially increase the circulating levels of insulin-like growth factor-1, growth hormone dependent binding proteins, thyroxine, tri-iodothyronine and testosterone. Active feedback inhibition loops prevent the target organs being overstimulated. The metabolism is altered in a beneficial way when growth hormone-secretagogues, thyrotropin-releasing hormone and gonadotropin-releasing hormone are administered together, whereas the effect of single-hormone treatment is minor and accompanied by side-effects. This new concept of a selectively reduced stimulation of pituitary function in the chronic phase of critical illness unveils new therapeutic perspectives to reverse the paradoxical wasting syndrome' and intensive care dependency.

Treatment with growth hormone and insulin-like growth factor-I in critical illness

The wider availability of recombinant human growth hormone and insulin-like growth factor-I has resulted in an investigation into the potential benefits of the pharmacological administration of these anabolic peptides in a variety of clinical conditions, characterized by an increase in catabolic rate. The initial studies were small, often uncontrolled open investigations, but investigators have more recently concentrated on larger, controlled multi-centre trials. Studies to date have included patients with cardiac failure, sepsis, burns, cancer cachexia, end-stage renal failure, trauma and AIDS, and those prior to or following major surgery. The authors have in general cautiously interpreted positive effects of treatment with growth hormone and insulin-like growth factor-I, either alone or in combination, on net protein balance, body composition, well-being and performance. Two large, randomized, placebo-controlled European multi-centre studies have recently detailed the effects of growth hormone treatment in critically ill intensive care patients. Major increases in mortality and morbidity were associated with growth hormone treatment. The mechanism(s) accounting for the increased mortality remain poorly understood. These negative findings have led to a decrease in the clinical use of growth hormone and in research activity in the area of anabolic treatment in human illness.

Five-day pulsatile gonadotropin-releasing hormone administration unveils combined hypothalamic-pituitary-gonadal defects underlying profound hypoandrogenism in men with prolonged critical illness

Central hyposomatotropism and hypothyroidism have been inferred in long-stay intensive care patients. Pronounced hypoandrogenism presumably also contributes to the catabolic state of critical illness. Accordingly, the present study appraises the mechanism(s) of failure of the gonadotropic axis in prolonged critically ill men by assessing the effects of pulsatile GnRH treatment in this unique clinical context. To this end, 15 critically ill men (mean +/- SD age, 67 +/- 12 yr; intensive care unit stay, 25 +/- 9 days) participated, with baseline values compared with those of 50 age- and BMI-matched healthy men. Subjects were randomly allocated to 5 days of placebo or pulsatile iv GnRH administration (0.1 microg/kg every 90 min). LH, GH, and TSH secretion was quantified by deconvolution analysis of serum hormone concentration-time series obtained by sampling every 20 min from 2100-0600 h at baseline and on nights 1 and 5 of treatment. Serum concentrations of gonadal and adrenal steroids, T(4), T(3), insulin-like growth factor I (IGF), and IGF-binding proteins as well as circulating levels of cytokines and selected metabolic markers were measured. During prolonged critical illness, pulsatile LH secretion and mean LH concentrations (1.8 +/- 2.2 vs. 6.0 +/- 2.2 IU/L) were low in the face of extremely low circulating total testosterone (0.27 +/- 0.18 vs. 12.7 +/- 4.07 nmol/L; P < 0.0001) and relatively low estradiol (E(2); 58.3 +/- 51.9 vs. 85.7 +/- 18.6 pmol/L; P = 0.009) and sex hormone-binding globulin (39.1 +/- 11.7 vs. 48.6 +/- 27.8 nmol/L; P = 0.01). The molar ratio of E(2)/T was elevated 37-fold in ill men (P < 0.0001) and correlated negatively with the mean serum LH concentrations (r = -0.82; P = 0.0002). Pulsatile GH and TSH secretion were suppressed (P < or = 0.0004), as were mean serum IGF-I, IGF-binding protein-3, and acid-labile subunit concentrations; thyroid hormone levels; and dehydroepiandrosterone sulfate. Morning cortisol was within the normal range. Serum interleukin-1beta concentrations were normal, whereas interleukin-6 and tumor necrosis factor-alpha were elevated. Serum tumor necrosis factor-alpha was positively correlated with the molar E(2)/testosterone ratio and with type 1 procollagen; the latter was elevated, whereas osteocalcin was decreased. Ureagenesis and breakdown of bone were increased. C-Reactive protein and white blood cell counts were elevated; serum lactate levels were normal. Intermittent iv GnRH administration increased pulsatile LH secretion compared with placebo by an increment of +8.1 +/- 8.1 IU/L at 24 h (P = 0.001). This increase was only partially maintained after 5 days of treatment. GnRH pulses transiently increased serum testosterone by +174% on day 2 (P = 0.05), whereas all other endocrine parameters remained unaltered. GnRH tended to increase type 1 procollagen (P = 0.06), but did not change serum osteocalcin levels or bone breakdown. Ureagenesis was suppressed (P < 0.0001), and white blood cell count (P = 0.0001), C-reactive protein (P = 0.03), and lactate level (P = 0.01) were increased by GnRH compared with placebo infusions. In conclusion, hypogonadotropic hypogonadism in prolonged critically ill men is only partially overcome with exogenous iv GnRH pulses, pointing to combined hypothalamic-pituitary-gonadal origins of the profound hypoandrogenism evident in this context. In view of concomitant central hyposomatotropism and hypothyroidism, evaluating the effectiveness of pulsatile GnRH intervention together with GH and TSH secretagogues will be important.

Therapeutic Options for the Treatment of Impaired Gut Function

Abstract. Tissue hypoxia, especially in the splanchnic area, is still considered to be an important cofactor in the pathogenesis of multiple organ failure. Therefore, the specific effects of the various therapeutic interventions on splanchnic perfusion and oxygenation are of particular interest. Restoring and maintaining oxygen transport and tissue oxygenation is the most important step in the supportive treatment of patients with sepsis and impaired gut perfusion. Therefore, supportive treatment should be focused on an adequate volume resuscitation and appropriate use of vasoactive drugs. Adequate volume loading may be the most important step in the treatment of patients with septic shock. An elevated oxygen delivery may be beneficial in some patients, but the increase of oxygen delivery should be guided by the measurement of parameters assessing global and regional oxygenation. Forcing an elevation in oxygen delivery by the use of very high dosages of catecholamines can be harmful. Vasopressors should be used for achieving an adequate perfusion pressure. For norepinephrine, no negative effects on gut perfusion have been demonstrated. Epinephrine and dopamine should be avoided because they seem to redistribute blood flow away from the splanchnic region. There are no convincing data yet to support the routine use of low-dose dopamine or dopexamine to improve an impaired gut perfusion. There is even evidence that low-dose dopamine may reduce the mucosal perfusion in the gut in some patients. It has been suggested that dopexamine can improve splanchnic perfusion, but because these effects remain somewhat controversial, a general recommendation for dopexamine to improve gut perfusion is not justified.

Pathophysiologic Features and Prevention of Human and Experimental Acute Tubular Necrosis

Abstract.Acute renal failure (ARF) remains a common and potentially devastating disorder that affects as many as 5% of all hospitalized patients, with a higher prevalence in patients in critical care units. The focus of this article is on categorizing recent pathophysiologic and clinically relevant developments in the field. The vascular and tubular factors in the pathogenesis of ARF, together with the potential mechanisms of recovery and repair of the injured kidney, are discussed. A number of experimental and clinical interventions to prevent. ARF are summarized. Although the clinical treatment of these patients is still largely supportive and many recent clinical trials showed rather negative results, it is hoped that basic research will provide therapeutic tools to improve the grim prognosis of this disease in the future.

Low-dose dopamine in patients with early renal dysfunction: a placebo-controlled randomised trial. Australian and New Zealand Intensive Care Society (ANZICS) Clinical Trials Group

BACKGROUND

Low-dose dopamine is commonly administered to critically ill patients in the belief that it reduces the risk of renal failure by increasing renal blood flow. However, these effects have not been established in a large randomised controlled trial, and use of dopamine remains controversial. We have done a multicentre, randomised, double-blind, placebo-controlled study of low-dose dopamine in patients with at least two criteria for the systemic inflammatory response syndrome and clinical evidence of early renal dysfunction (oliguria or increase in serum creatinine concentration).

METHODS

328 patients admitted to 23 participating intensive-care units (ICUs) were randomly assigned a continuous intravenous infusion of low-dose dopamine (2 microg kg(-1) min(-1)) or placebo administered through a central venous catheter while in the ICU. The primary endpoint was the peak serum creatinine concentration during the infusion. Analyses excluded four patients with major protocol violations.

FINDINGS

The groups assigned dopamine (n=161) and placebo (n=163) were similar in terms of baseline characteristics, renal function, and duration of trial infusion. There was no difference between the dopamine and placebo groups in peak serum creatinine concentration during treatment (245 [SD 144] vs 249 [147] micromol/L; p=0.93), in the increase from baseline to highest value during treatment (62 [107] vs 66 [108] micromol/L; p=0.82), or in the numbers of patients whose serum creatinine concentration exceeded 300 micromol/L (56 vs 56; p=0.92) or who required renal replacement therapy (35 vs 40; p=0.55). Durations of ICU stay (13 [14] vs 14 [15] days; p=0.67) and of hospital stay (29 [27] vs 33 [39] days; p=0.29) were also similar. There were 69 deaths in the dopamine group and 66 in the placebo group.

INTERPRETATION

Administration of low-dose dopamine by continuous intravenous infusion to critically ill patients at risk of renal failure does not confer clinically significant protection from renal dysfunction.

The metabolic and nutritional response to critical illness

The metabolic response to critical illness promotes catabolism, which mobilizes substrates for energy. Initially the hypothalamic-pituitary-adrenal axis is stimulated, but later there appears to be anterior pituitary depression. Despite this, the early increase in plasma cortisol levels is usually maintained by means independent of (falling) corticotropin levels. Some patients, however, develop acute adrenal insufficiency and appear to benefit from replacement exogenous glucocorticoid. However, identifying such patients is often difficult. The replacement of other deficiencies may not be in the patients' interests. For example, leptin, a stress-related hormone, has multiple effects, some seemingly advantageous and others detrimental in critical illness. Its overall influence and significance remains unclear.The health of gut mucosa and the inflammatory response might be improved or influenced to the (presumed) benefit of the patient by agents such as glutamine, arginine, some eicosanoids, and exogenous nucleic acids. Such "immunonutrition" appears to improve mortality and other measures of outcome in surgical intensive care unit patients and those with sepsis.

Thyroid function in very low birthweight infants after intravenous administration of the iodinated contrast medium iopromide

BACKGROUND

Thyroid function disorders have often been observed in preterm infants after intravenous administration of iodinated contrast medium. The effect on thyroid function depends on the dosage, but the choice of the contrast medium may be equally important, as there are appreciable pharmacological differences between them.

METHOD

Thyroid function was analysed in 20 very low birthweight infants of gestational age less than 30 weeks after injection of iopromide, a monomeric non-ionic iodinated contrast medium. Levels of free thyroxine and thyroid stimulating hormone were compared with those in 26 control infants.

RESULTS

Free thyroxine levels in all study infants ranged from 9.0 to 25.7 pmol/l (days 14-21) and 9.0 to 23.2 pmol/l (days 35-49), and thyroid stimulating hormone levels ranged from 0.13 to 0.26 mU/l (days 14-21) and 0.26 to 11.11 mU/l (days 35-49). These levels were not altered after injection of iopromide.

CONCLUSION

The risk of transient hypothyroidism or hyperthyrotropinaemia may be reduced with the use of iopromide compared with other contrast media.

Lack of renoprotective effects of dopamine and furosemide during cardiac surgery

Because development of acute renal failure is one of the most potent predictors of outcome in cardiac surgery patients, the prevention of renal dysfunction is of utmost importance in perioperative care. In a double-blind randomized controlled trial, the effectiveness of dopamine or furosemide in prevention of renal impairment after cardiac surgery was evaluated. A total of 126 patients with preoperatively normal renal function undergoing elective cardiac surgery received a continuous infusion of either "renal-dose" dopamine (2 microg/kg per min) (group D), furosemide (0.5 microg/kg per min) (group F), or isotonic sodium chloride as placebo (group P), starting at the beginning of surgery and continuing for 48 h or until discharge from the intensive care unit, whichever came first. Renal function parameters and the maximal increase of serum creatinine above baseline value within 48 h (deltaCrea(max)) were determined. The increase in plasma creatinine was twice as high in group F as in groups D and P (P < 0.01). Acute renal injury (defined as deltaCreamax) >0.5 mg/dl) occurred more frequently in group F (six of 41 patients) than in group D (one of 42) and group P (zero of 40) (P < 0.01). (The difference between group D and group P was not significant.) Creatinine clearance was lower in group F (P < 0.05). Two patients in group F required renal replacement therapy. The mean volume of infused fluids, blood urea nitrogen, serum sodium, serum potassium, and osmolar- and free-water clearance was similar in all groups. It was shown that continuous infusion of dopamine for renal protection was ineffective and was not superior to placebo in preventing postoperative dysfunction after cardiac surgery. In contrast, continuous infusion of furosemide was associated with the highest rate of renal impairment. Thus, renaldose dopamine is ineffective and furosemide is even detrimental in the protection of renal dysfunction after cardiac surgery.

Metoclopramide: a novel adjunct for improving cardiac and hepatocellular functions after trauma-hemorrhage

Although metoclopramide (MCP) administration after trauma-hemorrhage restores the depressed immune functions, it remains unknown whether this agent has any salutary effects on the depressed cardiovascular and hepatocellular functions under those conditions. Adult male Sprague-Dawley rats underwent a midline laparotomy (i.e., induction of soft-tissue trauma) and were then bled to and maintained at a mean arterial pressure (MAP) of 40 mmHg until 40% of the maximal shed blood volume was returned in the form of Ringer lactate (RL). The rats were then resuscitated with four times the shed blood volume in the form of RL over 60 min. MCP (2 mg/kg body wt) or vehicle was administered subcutaneously at the end of resuscitation. At 24 h after resuscitation, cardiac index and hepatocellular function (i.e., the maximum velocity and the overall efficiency of indocyanine green clearance) were determined. Plasma levels of interleukin (IL)-6 and prolactin were also assayed. The results indicate that treatment with MCP after trauma-hemorrhage and resuscitation significantly improved the depressed cardiac output and hepatocellular function. Furthermore, MCP administration significantly increased circulating levels of prolactin and decreased the plasma levels of the proinflammatory cytokine IL-6. Thus, administration of MCP, which increased prolactin secretion, appears to be a useful adjunct for restoring the depressed cardiac and hepatocellular functions and downregulating inflammatory cytokine release after trauma and hemorrhagic shock.

Nitric oxide scavengers in the treatment of shock associated with systemic inflammatory response syndrome

Shock associated with systemic inflammatory response syndrome (SIRS) is a form of distributive shock affecting over 200,000 patients per year in the US that results in 50% mortality. The role of NO in a variety of shock states has been extensively studied and has been shown to be the primary effector in endotoxin lipopolysaccharide (LPS)-induced hypotension attendant to shock associated with sepsis or presumed sepsis. NO has also been shown to be a myocardial depressant, an inhibitor of mitochondrial electron transport, an inducer of vascular leakage and an enhancer of LPS-induced cytokine release. Hence, it is involved in a wide variety of shock-related pathologies and is a key target for therapeutic intervention. Mechanism of action based therapies using NO scavengers represent a promising new approach. However, because NO is involved in such a wide variety of both physiological and pathophysiological processes, a therapy directed at NO must be selective in order to be both safe and effective. NO scavengers are comprised of two basic classes: organic molecules and metal complexes. Pyridoxalated haemoglobin polyoxyethylene conjugate (PHP) is a chemically-modified haemoglobin. It is the furthest advanced NO scavenger in clinical trials and is about to enter pivotal Phase 3 trials in patients with shock associated with SIRS.

Dopexamine increases splanchnic blood flow but decreases gastric mucosal pH in severe septic patients treated with dobutamine

OBJECTIVE

To assess the effects of dopexamine on splanchnic blood flow and splanchnic oxygen uptake in septic patients.

DESIGN

A prospective, controlled trial.

SETTING

A ten-bed intensive care unit (ICU) in a university hospital.

PATIENTS

Twelve patients with severe sepsis (according to the criteria of the 1992 American College of Chest Physicians/Society of Critical Care Medicine consensus conference) being stabilized by volume loading and treated to an elevated oxygen delivery by dobutamine infusion.

INTERVENTIONS

Infusion of increasing dosages of dopexamine (0.5, 1.0, 2.0, and 4.0 microg/kg/min).

MEASUREMENTS AND MAIN RESULTS

Systemic and splanchnic hemodynamic and oxygen transport parameters as well as gastric mucosal pH (pHi) were measured. A hepatic venous catheter technique with indocyanine green dye dilution was used to determine splanchnic blood flow. Dopexamine increased global and splanchnic oxygen delivery without affecting oxygen consumption (VO2). Splanchnic blood flow increased proportionally to cardiac output, indicating that there was no selective effect of dopexamine on the splanchnic flow. Dopexamine decreased pHi in a dose-dependent fashion in all 12 patients.

CONCLUSIONS

In hemodynamically stable, hyperdynamic septic patients being treated with dobutamine, dopexamine has no selective effect on splanchnic blood flow. In fact, a decreased pHi suggests a harmful effect on gastric mucosal perfusion.

The effect of 'renal-dose' dopamine on renal tubular function following cardiac surgery: assessed by measuring retinol binding protein (RBP)

OBJECTIVE

Acute renal failure complicating open heart surgery is not uncommon. Dopamine infusion (2.5-4.0 microg/kg per min) has often been advocated for prophylactic 'renal protection' in this setting despite little objective evidence of real benefit. We aimed to investigate whether dopamine offers any 'renal protection' in patients with normal heart and kidney functions undergoing routine coronary artery bypass grafting (CABG). Urinary excretion of retinol-binding protein (RBP), previously validated as a sensitive and accurate marker of early renal tubular injury, was used to assess the renal effects of dopamine during the first postoperative week.

METHODS

Forty consecutive patients from the elective waiting list were prospectively randomized into two equal groups: those in Group A received dopamine infusion at 'renal dose' (2.5-4.0 microg/kg per min) starting from induction of anaesthesia for 48 h, whereas those in Group B served as untreated controls. Daily measurements were made of weight-adjusted urine output (ml/kg), fluid balance (input/output), serum creatinine, blood urea and urinary RBP. Statistical comparisons were made using Mann-Whitney U-test.

RESULTS

The two groups matched in terms of age, time and temperature on cardiopulmonary bypass, number of grafts performed and perioperative haemodynamic status. No differences were detected in the weight-adjusted urine output, fluid balance, serum creatinine and blood urea between the groups. Control subjects (Group B) showed an increase in urinary RBP during the first and second postoperative days (323+/-4 microg/ mmolCr and 50+/-3 microg/mmolCr; mean+/-SD). However, patients treated with dopamine (Group A) demonstrated much greater urinary excretion of RBP over the same period (1257+/-15 microg/mmolCr and 449+/-21 microg/mmolCr; P = 0.0006 and 0.03) than those in Group B.

CONCLUSIONS

Dopamine given at 'renal-dose' appears to offer no renal protection in patients with normal heart and kidney functions undergoing elective coronary surgery. On the contrary, it exacerbates the severity of renal tubular injury during the early postoperative period. Based on these findings we do not recommend the use of dopamine for routine renal prophylaxis in this group of patients.

Can the use of low-dose dopamine for treatment of acute renal failure be justified?

The use of dopamine for the prevention and treatment of acute renal failure is widespread. Its use is based on physiology suggesting selective renal vasodilation when it is infused at low dose. This article reviews the available data on the clinical use of dopamine. When used to prevent acute renal failure in high-risk treatments there is no evidence of benefit of dopamine but, given the low incidence of significant renal failure, the studies are underpowered. In treatment of acute renal failure, the quality of the data is poor. Only in one small randomised trial of moderate acute renal failure in patients with malaria was a clinically significant benefit of dopamine shown. The rest of the data, in the form of case series, showed either no benefit of dopamine or small benefits of little clinical significance. Again, these studies are of insufficient power for conclusions to be drawn as to the overall benefits and risks. We conclude that benefits of dopamine use cannot be ruled out by currently available data but its use cannot be advised until trials examining clinically important endpoints in large numbers of patients have been performed.

Reactivation of pituitary hormone release and metabolic improvement by infusion of growth hormone-releasing peptide and thyrotropin-releasing hormone in patients with protracted critical illness

Protracted critical illness is marked by protein wasting resistant to feeding, by accumulation of fat stores, and by suppressed pulsatile release of GH and TSH. We previously showed that the latter can be reactivated by brief infusion of GH-releasing peptide (GHRP-2) and TRH. Here, we studied combined GHRP-2 and TRH infusion for 5 days, which allowed a limited evaluation of the metabolic effectiveness of this novel trophic endocrine strategy. Fourteen patients (mean +/- SD age, 68 +/- 11 yr), critically ill for 40 +/- 28 days, were compared to a matched group of community-living control subjects at baseline and subsequently received 5 days of placebo and 5 days of GHRP-2 plus TRH (1 + 1 microg/kg x h) infusion in random order. At baseline, impaired anabolism, as indicated by biochemical markers (osteocalcin and leptin), was linked to hyposomatotropism [reduced pulsatile GH secretion, as determined by deconvolution analysis, and low GH-dependent insulin-like growth factor and binding protein (IGFBP) levels]. Biochemical markers of accelerated catabolism (increased protein degradation and bone resorption) were related to tertiary hypothyroidism and the serum concentration of IGFBP-1, but not to hyposomatotropism. Metabolic markers were independent of elevated serum cortisol. After 5 days of GHRP-2 plus TRH infusion, osteocalcin concentrations increased 19% vs. -6% with placebo, and leptin had rose 32% vs. -15% with placebo. These anabolic effects were linked to increased IGF-I and GH-dependent IGFBP, which reached near-normal levels from day 2 onward. In addition, protein degradation was reduced, as indicated by a drop in the urea/creatinine ratio, an effect that was related to the correction of tertiary hypothyroidism, with near-normal thyroid hormone levels reached and maintained from day 2 onward. Concomitantly, a spontaneous tendency of IGFBP-1 to rise and of insulin to decrease was reversed. Cortisol concentrations were not detectably altered. In conclusion, 5-day infusion of GHRP-2 plus TRH in protracted critical illness reactivates blunted GH and TSH secretion, with preserved pulsatility, peripheral responsiveness, and feedback inhibition and without affecting serum cortisol, and induces a shift toward anabolic metabolism. This provides the first evidence of the metabolic effectiveness of short term GHRP-2 plus TRH agonism in this particular wasting condition.

Supportive therapy of the sepsis syndrome

Adequate volume loading may be the most important step in the treatment of patients with septic shock. Techniques allowing us to achieve and tightly control volume loading and regional perfusion are considered to be helpful. An elevated oxygen delivery may be beneficial in some patients but the increase of oxygen delivery should be guided by the measurement of parameters assessing global and regional oxygenation. Forcing an increase in oxygen delivery by the use of very high dosages of catecholamines can be harmful. Vasopressors should be used for achieving an adequate perfusion pressure. For norepinephrine, no negative effects on regional perfusion have been demonstrated. Epinephrine and dopamine should be avoided because they seem to redistribute blood flow away from the splanchnic region. There are no convincing data yet to support the routine use of low dose dopamine or dopexamine in patients with sepsis. Neither low dose dopamine nor dopexamine has been proven to prevent renal failure in septic patients. Furthermore, there is evidence that low dose dopamine may reduce mucosal perfusion in the gut in some patients. There is some suggestion that dopexamine can improve splanchnic perfusion but since these effects remain somewhat controversial, there is no reason for a general recommendation for dopexamine in septic patients.

Pathophysiology of the neuroregulation of growth hormone secretion in experimental animals and the human

During the last decade, the GH axis has become the compelling focus of remarkably active and broad-ranging basic and clinical research. Molecular and genetic models, the discovery of human GHRH and its receptor, the cloning of the GHRP receptor, and the clinical availability of recombinant GH and IGF-I have allowed surprisingly rapid advances in our knowledge of the neuroregulation of the GH-IGF-I axis in many pathophysiological contexts. The complexity of the GHRH/somatostatin-GH-IGF-I axis thus commends itself to more formalized modeling (154, 155), since the multivalent feedback-control activities are difficult to assimilate fully on an intuitive scale. Understanding the dynamic neuroendocrine mechanisms that direct the pulsatile secretion of this fundamental growth-promoting and metabolic hormone remains a critical goal, the realization of which is challenged by the exponentially accumulating matrix of experimental and clinical data in this arena. To the above end, we review here the pathophysiology of the GHRH somatostatin-GH-IGF-I feedback axis consisting of corresponding key neurotransmitters, neuromodulators, and metabolic effectors, and their cloned receptors and signaling pathways. We propose that this system is best viewed as a multivalent feedback network that is exquisitely sensitive to an array of neuroregulators and environmental stressors and genetic restraints. Feedback and feedforward mechanisms acting within the intact somatotropic axis mediate homeostatic control throughout the human lifetime and are disrupted in disease. Novel effectors of the GH axis, such as GHRPs, also offer promise as investigative probes and possible therapeutic agents. Further understanding of the mechanisms of GH neuroregulation will likely allow development of progressively more specific molecular and clinical tools for the diagnosis and treatment of various conditions in which GH secretion is regulated abnormally. Thus, we predict that unexpected and enriching insights in the domain of the neuroendocrine pathophysiology of the GH axis are likely be achieved in the succeeding decades of basic and clinical research.

Low IGF-I levels are often uncoupled with elevated GH levels in catabolic conditions

Increased GH together with decreased IGF-I levels pointing to peripheral GH insensitivity in critically ill patients have been reported by some but not by other authors. To clarify whether elevated GH levels are coupled with low IGF-I levels in all catabolic conditions, basal GH and IGF-I levels were evaluated in patients with sepsis (SEP, no.=13; age [mean+/-SE]=59.2+/-1.2 yr), trauma (TRA, no.=16; age=42.3+/-3.4 yr), major burn (BUR, no.=26; age=52.8+/-4.2 yr) and post-surgical patients (SUR, no.=11; age=55.0+/-4.7 yr) 72 hours after ICU admission or after cardiac surgery. GH and IGF-I levels were also evaluated in normal subjects (NS, no.=75; age=44.0+/-1.5 yr), in adult hypopituitaric patients with severe GH deficiency (GHD, no.=54; age=44.8+/-2.3 yr), in patients with liver cirrhosis (LC, no.=12; age=50.4+/-2.8 yr) and in patients with anorexia nervosa (AN, no.=19; age=18.7+/-0.8 yr). Basal IGF-I and GH levels in GHD were lower than in NS (68.6+/-6.4 vs 200.9+/-8.7 microg/l and 0.3+/-0.1 vs 1.4+/-0.2 microg/l; p<0.01). On the other hand, AN and LC showed IGF-I levels (70.4+/-9.1 and 52.4+/-10.5 microg/l) similar to those in GHD while GH levels (10.0+/-2.8 and 7.9+/-2.1 microg/l) were higher than those in NS (p<0.01). IGF-I levels in SEP (84.5+/-8.8 microg/l) were similar to those in GHD, AN and LC and lower than those in NS (p<0.01). IGF-I levels in BUR (105.2+/-10.9 microg/l) were lower than in NS (p<0.01) but higher than those in GHD, AN, LC and SEP (p<0.01). On the other hand, in TRA (162.8+/-17.4 microg/l) and SUR (135.0+/-20.7 microg/l) IGF-I levels were lower but not significantly different from those in NS and clearly higher than those in GHD, AN, LC, SEP and BUR. Basal GH levels in SEP (0.6+/-0.2 microg/l), TRA (1.8+/-0.5 microg/l), SUR (2.2+/-0.5 microg/l) and BUR (2.2+/-0.5 microg/l) were similar to those in NS, higher (p<0.05) than those in GHD and lower (p<0.01) than those in AN and LC. In conclusion, our data demonstrate that low IGF-I levels are not always coupled with elevated GH levels in all catabolic conditions. Differently from cirrhotic and anorectic patients, in burned and septic patients GH levels are not elevated in spite of very low IGF-I levels similar to those in panhypopituitaric GHD patients. These findings suggest that in some catabolic conditions peripheral GH insensitivity and somatotrope insufficiency could be concomitantly present.

Persistent endocrine stress response in patients undergoing cardiac surgery

To investigate the endocrine stress response in patients undergoing major surgery with general anesthesia using a balanced technique with sufentanil, isoflurane and midazolam up to the second postoperative day, blood levels of cortisol, epinephrine, norepinephrine, prolactin and growth hormone were determined in 68 males for elective coronary artery bypass grafting (CABG) surgery. Intraoperatively, during extracorporeal circulation none of the measured parameters were significantly increased compared to preoperative values. The endocrine response of patients with perioperative epinephrine medication (n = 32) was not significant different to patients that did not receive exogenous epinephrine (n = 36). On the evening of the day of surgery, levels of cortisol (3 fold), epinephrine (4.7 fold), norepinephrine (1.7 fold) and growth hormone (16.5 fold) were significantly increased. Compared to preoperative values levels of cortisol (3.3 fold), growth hormone (5.5 fold) and norepinephrine (1.8 fold) remained elevated up to the evening of the second postoperative day. In conclusion, the endocrine stress response in patients undergoing CABG-surgery under general anesthesia with sufentanil, midazolam, isoflurane is intraoperatively prevented by anesthesia. Although hemodilution or hormone degradation might be responsible for the lack of an increase in endocrine parameters during CPB, this study indicates that a balanced technique with isoflurane, sufentanil and midazolam is more effective in blocking the endocrine stress response than previously described anesthetic techniques. In the early postoperative period, a sharp increase in cortisol, epinephrine, norepinephrine and growth hormone occurred suggesting that the predominant endocrine stress response begins in the intensive care unit with end of anesthesia. The postoperative elevated levels of cortisol, growth hormone and norepinephrine indicate a persisting stress-response for more than two days after surgical trauma.