Hypoglycaemia in Clinical Diabetes

Hypoglycaemia in Clinical Diabetes

(Parte 3 de 7)

Catecholamines also reduce glucose utilisation peripherally and inhibit insulin secretion. Cortisol and growth hormone increase gluconeogenesis and reduce glucose utilisation. The role of the other hormones (see below) in counterregulation is unclear, but they are unlikely to make a significant contribution. Finally, there is evidence that during profound hypoglycaemia (blood glucose below 1.7mmol/l), hepatic glucose output is stimulated directly, although the mechanism is unknown. This is termed hepatic autoregulation.

The depth, as well as the duration, of hypoglycaemia is important in determining the magnitude of the counterregulatory hormone response. Studies using ‘hyperinsulinaemic clamps’ show a hierarchical response of hormone production. In this technique, insulin is infused at a constant rate and a glucose infusion rate varied to maintain blood glucose concentrations within ±0 2mmol/l of target concentrations. This permits the controlled evaluation of the counterregulatory hormone response at varying degrees of hypoglycaemia. It also demonstrates that glucagon, catecholamines and growth hormone start to be secreted at a blood glucose concentration of 3.5–3.7mmol/l, with cortisol produced at a lower glucose of 3.0mmol/l (Mitrakou et al., 1991). The counterregulatory response is initiated before impairment in cerebral function commences, usually at a blood glucose concentration of approximately 3.0mmol/l (Heller and Macdonald, 1996).

The magnitude of the hormonal response also depends on the length of the hypoglycaemic episode. The counterregulatory hormonal response commences up to 20 minutes after hypoglycaemia is achieved and continues to rise for 60 minutes (Kerr et al., 1989). In contrast, this response is attenuated as a result of a previous episode of hypoglycaemia (within a few days) (reviewed by Heller and Macdonald, 1996) and even by prolonged exercise the day before hypoglycaemia is induced. Galassetti et al. (2001) showed that in non-diabetic subjects three hours of moderate intensity exercise the previous day markedly decreased the counterregulatory response to hypoglycaemia induced by the infusion of insulin, and that the reduced counterregulatory response was more marked in men than in women.

Although the primary role of the counterregulatory hormones is on glucose metabolism, any effects on fatty acid utilisation can have an indirect effect on blood glucose. Thus, the increase in plasma epinephrine (adrenaline) (and activation of the sympathetic nervous system) that is seen in hypoglycaemia can stimulate lipolysis of triglyceride in adipose tissue and muscle and release fatty acids which can be used as an alternative fuel to glucose, making more glucose available for the CNS. Enoksson et al. (2003) demonstrated that patients with type 1 diabetes, who had lower plasma epinephrine responses to hypoglycaemia than nondiabetic controls, also had reduced rates of lipolysis in adipose tissue and skeletal muscle, making them more dependent on glucose as a fuel and therefore at risk of developing a more severe hypoglycaemia.

The complex counterregulatory and homeostatic mechanisms described above are thought to be mostly under the control of the central nervous system. Evidence for this comes from studies in dogs, where glucose was infused into the carotid and vertebral arteries to maintain euglycaemia in the brain. Despite peripheral hypoglycaemia, glucagon did not increase and responses of the other counterregulatory hormones were blunted. This, and

10 NORMAL GLUCOSE METABOLISM AND RESPONSES other studies in rats, led to the hypothesis that the ventromedial nucleus of the hypothalamus (VMH), which does not have a blood–brain barrier, acts as a glucose-sensor and co-ordinates counterregulation (Borg et al., 1997). However, evidence exists that other parts of the brain may also be involved in mediating counterregulation.

It is now clear that glucose-sensing neurones can involve either glucokinase or ATP- sensitive K+ channels (Levin et al., 2004). In rats, the VMH has ATP-sensitive K+ channels which seem to be involved in the counterregulatory responses to hypoglycaemia, as injection of the sulphonylurea, glibenclamide, directly into the VMH suppressed hormonal responses to systemic hypoglycaemia (Evans et al., 2004).

The existence of hepatic autoregulation suggests that some peripheral control should exist.

Studies producing central euglycaemia and hepatic portal venous hypoglycaemia in dogs have provided evidence for hepatic glucose sensors and suggest that these sensors, as well as those in the brain, are important in the regulation of glucose (Hamilton-Wessler et al., 1994). However, this topic is somewhat controversial and more recent studies on dogs have failed to demonstrate an effect of hepatic sensory nerves on the responses to hypoglycaemia (Jackson et al., 2000). Moreover, studies in humans by Heptulla et al. (2001) showed that providing glucose orally rather than intravenously during a hypoglycaemic hyperinsulinaemic clamp actually enhanced the counterregulatory hormone responses rather than reduced them.

Hypoglycaemia induces the secretion of various hormones, some of which are responsible for counterregulation, many of the physiological changes that occur as a consequence of lowering blood glucose and contribute to symptom generation (see Chapter 2), The stimulation of the autonomic nervous system is central to many of these changes.

Activation of the Autonomic Nervous System

The autonomic nervous system comprises sympathetic and parasympathetic components (Figure 1.5). Fibres from the sympathetic division leave the spinal cord with the ventral roots from the first thoracic to the third or fourth lumbar nerves to synapse in the sympathetic chain or visceral ganglia, and the long postganglionic fibres are incorporated in somatic nerves. The parasympathetic pathways originate in the nuclei of cranial nerves I, VII, IX and X, and travel with the vagus nerve. A second component, the sacral outflow, supplies the pelvic viscera via the pelvic branches of the second to fourth spinal nerves. The ganglia in both cases are located near the organs supplied, and the postganglionic neurones are therefore short.

Selective activation of both components of the autonomic system occurs during hypoglycaemia. The sympathetic nervous system in particular is responsible for many of the physiological changes during hypoglycaemia and the evidence for its activation can be obtained indirectly by observing functional changes such as cardiovascular responses (considered below), measuring plasma catecholamines which gives a general index of sympathetic activation, or by directly recording sympathetic activity.

HORMONAL CHANGES DURING HYPOGLYCAEMIA 1

Figure 1.5 Anatomy of the autonomic nervous system. Pre=preganglionic neurones; post=postganglionic neurones; RC=ramus communicans

Direct recordings are possible from sympathetic nerves supplying skeletal muscle and skin.

Sympathetic neural activity in skeletal muscle involves vasoconstrictor fibres which innervate blood vessels and are involved in controlling blood pressure. During hypoglycaemia (induced by insulin), the frequency and amplitude of muscle sympathetic activity are increased as blood glucose falls, with an increase in activity eight minutes after insulin is injected intravenously, peaking at 25–30 minutes coincident with the glucose nadir, and persisting

12 NORMAL GLUCOSE METABOLISM AND RESPONSES

Figure 1.6 (a) Muscle sympathetic activity during euglycaemia and hypoglycaemia. Reproduced from Fagius et al. (1986) courtesy of the American Diabetes Association. (b) Skin sympathetic activity during euglycaemia and hypoglycaemia. Reproduced from Berne and Fagius (1986), with kind permission from Springer Science and Business Media for 90 minutes after euglycaemia is restored (Figure 1.6a) (Fagius et al., 1986). During hypoglycaemia, a sudden increase in skin sympathetic activity is seen, which coincides with the onset of sweating. This sweating leads to vasodilatation of skin blood vessels, which is also contributed to by a reduction in sympathetic stimulation of the vasoconstrictor components of skin arterio-venous anastomoses (Figure 1.6b) (Berne and Fagius, 1986). These effects (at least initially) increase total skin blood flow and promote heat loss from the body.

Activation of both muscle and skin sympathetic nerve activity are thought to be centrally mediated. Tissue neuroglycopenia can be produced by 2-deoxy-D-glucose, a glucose analogue, without increasing insulin. Infusion of this analogue causes stimulation of muscle and skin sympathetic activity demonstrating that it is the hypoglycaemia per se, and not the insulin used to induce it, which is responsible for the sympathetic activation (Fagius and Berne, 1989).

The activation of the parasympathetic nervous system (vagus nerve) during hypoglycaemia cannot be measured directly. The most useful index of parasympathetic function is the measurement of plasma pancreatic polypeptide, the peptide hormone secreted by the P cells of the pancreas, which is released in response to vagal stimulation.

HORMONAL CHANGES DURING HYPOGLYCAEMIA 13

Neuroendocrine Activation (Box 1.3)

Insulin-induced hypoglycaemia was used to study pituitary function as early as the 1940s. The development of assays for adrenocorticotrophic hormone (ACTH) and growth hormone (GH) allowed the direct measurement of pituitary function during hypoglycaemia in the 1960s, and many of the processes governing these changes were unravelled before elucidation of the counterregulatory system. The studies are comparable to those evaluating counterregulation, in that potential regulatory factors are blocked to measure the hormonal response to hypoglycaemia with and without the regulating factor.

Box 1.3 Neuroendocrine activation

Hypothalamus ↑ Corticotrophic releasing hormone ↑ Growth hormone releasing hormone

Anterior Pituitary ↑ Adrenocorticotrophic hormone ↑ Beta endorphin

↑ Growth hormone

↑ Prolactin ↔ Thyrotrophin

↔ Gonadotrophins

Posterior pituitary ↑ Vasopressin ↑ Oxytocin

Pancreas ↑ Glucagon ↑ Pancreatic polypeptide

↑ Insulin

Adrenal ↑ Cortisol ↑ Epinephrine (adrenaline)

↑ Aldosterone

Others ↑ Parathyroid hormone ↑ Gastrin

Hypothalamus and anterior pituitary

ACTH, GH and prolactin concentrations increase during hypoglycaemia, but there is no change in thyrotrophin or gonadotrophin secretion. The secretion of these pituitary hormones is controlled by releasing factors which are produced in the median eminence of the hypothalamus, secreted into the hypophyseal portal vessels and then pass to the pituitary gland (Figure 1.7). The mechanisms regulating the releasing factors are incompletely understood, but may involve the ventromedial nucleus, one site where brain glucose sensors are situated (Fish et al., 1986).

14 NORMAL GLUCOSE METABOLISM AND RESPONSES

Figure 1.7 Anatomy of the hypothalamus and pituitary gland

• ACTH: Secretion is governed by release of corticotrophin releasing hormone (CRH) from the hypothalamus; alpha adrenoceptors stimulate CRH release, and beta adrenoceptors have an inhibitory action. A variety of neurotransmitters control the release of CRH into the portal vessels, including serotonin and acetylcholine which are stimulatory and GABA which is inhibitory. The increase in ACTH causes cortisol to be secreted from the cortices of the adrenal glands.

• Beta endorphins are derived from the same precursors as ACTH and are co-secreted with it. The role of endorphins in counterregulation is uncertain, but they may influence the secretion of the other pituitary hormones during hypoglycaemia.

• GH: Growth hormone secretion is governed by two hypothalamic hormones: growth hormone releasing hormone (GHRH) which stimulates GH secretion, and somatostatin which is inhibitory. GHRH secretion is stimulated by dopamine, GABA, opiates and through alpha adrenoceptors, whereas it is inhibited by serotonin and beta adrenoceptors. A study in rats showed that bioassayable GH and GHRH are depleted in the pituitary and hypothalamus respectively after insulin-induced hypoglycaemia (Katz et al., 1967).

• Prolactin: The mechanisms underlying its secretion are not established. Prolactin secretion is normally under the inhibitory control of dopamine, but evidence also exists for releasing factors being produced during hypoglycaemia. Prolactin does not contribute to counterregulation.

Posterior pituitary

Vasopressin and oxytocin both increase during hypoglycaemia (Fisher et al., 1987). Their secretion is under hormonal and neurotransmitter control in a similar way to the hypothalamic

PHYSIOLOGICAL RESPONSES 15 hormones. Vasopressin has glycolytic actions and oxytocin increases hepatic glucose output in dogs, but their contribution to glucose counterregulation is uncertain.

Pancreas

• Glucagon: The mechanisms of glucagon secretion during hypoglycaemia are still not fully understood. Although activation of the autonomic nervous system stimulates its release, this pathway has been shown to be less important in humans. A reduction in glucose concentrations may have a direct effect on the glucagon-secreting pancreatic alpha cells, or the reduced beta cell activity (reduced insulin secretion), which also occurs with low blood glucose, may release the tonic inhibition of glucagon secretion. However, such mechanisms would be disturbed in type 1 diabetes, where hypoglycaemia is normally associated with high plasma insulin levels and there is no direct effect of beta cell-derived insulin on the alpha cells.

• Somatostatin: This is thought of as a pancreatic hormone produced from D cells of the islets of Langerhans, but it is also secreted in other parts of the gastrointestinal tract. There are a number of structurally different polypeptides derived from prosomatostatin: the somatostatin-14 peptide is secreted from D cells, and somatostatin-28 from the gastrointestinal tract. The plasma concentration of somatostatin-28 increases during hypoglycaemia (Francis and Ensinck, 1987). The normal action of somatostatin is to inhibit the secretion both of insulin and glucagon, but somatostatin-28 inhibits insulin ten times more effectively than glucagon, and thus may have a role in counterregulation by suppressing insulin release.

• Pancreatic polypeptide: This peptide has no known role in counterregulation, but its release during hypoglycaemia is stimulated by cholinergic fibres through muscarinic receptors and is a useful marker of parasympathetic activity.

Adrenal and Renin–Angiotensin system

The processes governing the increase in cortisol during hypoglycaemia are discussed above. The rise in catecholamines, in particular epinephrine from the adrenal medulla, which occurs when blood glucose is lowered, is controlled by sympathetic fibres in the splanchnic nerve. The increase in renin, and therefore angiotensin and aldosterone, during hypoglycaemia is stimulated primarily by the intra-renal effects of increased catecholamines, mediated through beta adrenoceptors, although the increase in ACTH and hypokalaemia due to hypoglycaemia contributes (Trovati et al., 1988; Jungman et al., 1989). These changes do not have a significant role in counterregulation, although angiotensin I has glycolytic actions in vitro.

(Parte 3 de 7)

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