INYECCIÓN

The changes in bladder function in DM have been attributed to autonomic neuropathy, changes in autonomic receptors and alterations in detrusor muscle structure and function.

16.1 Autonomic neuropathy

Diabetic neuropathy affects spinal nerve roots, sympathetic ganglia, peripheral autonomic nerves, intramural ganglia and nerve fibers o f the bladder wall (Jordan et al 1935, Bartley et al 1966, Ellenberg et al 1967, Faerman et al 1973, Mastri 1980). The pathological changes described include nerve loss, axonal regeneration and segmental demyelination (Brown et al 1984). Although the exact aetiology o f diabetic neuropathy is unclear, it is likely to be multifactorial. Early observations indicated that it was the result o f

atherosclerotic vascular disease (Woltman et al 1929). This hypothesis has now largely been discarded, although microvascular occlusion and infarction does appear to be related to some diabetic mononeuropathies (Raff et al 1968). More recent studies have implicated changes in the axonal blood supply (Low 1987, Cameron et al 1994), abnormal myoinositol metabolism (Low 1987) and alterations in the intracellular pathways induced by chronic hyperglycaemia (Von-Poppel et al 1988, Green et al 1985) as important factors contributing to the pathogenesis o f diabetic neuropathy. The presence o f enhanced sorbitol concentration within nerves as a consequence o f increased aldose reductase activity may also contribute to the development o f diabetic autonomic neuropathy (Gabbay et al 1966). This is because sorbitol induces structural damage o f nerves and hence produces neuropathy (Green et al 1985). Not surprisingly, the aldose reductase inhibitor, sorbinil has been shown to enhance axonal transport o f glucose (Tomlinson et al 1984) and improve nerve conduction velocity (Yue et al 1987).

Impairment o f nerve blood flow is now considered to be an important factor in the pathogenesis o f diabetic neuropathy. The endoneural hypoxia is thought to be sufficient enough to cause alterations in neuronal conduction velocity. ET-1 has been shown to cause a significant reduction in blood flow to peripheral sensory nerves (Zochodone et al 1992) in rats, similar to that observed after induction o f DM (Cameron et al 1994). The ischemic/hypoxic-induced neuropathy is similar in DM and ET-1-treated animals, suggesting the enhanced local production o f ET-1 (or elevated circulating levels of ET-1 that is known to occur in DM (Takahashi K et al 1990)) can contribute to the pathogenesis o f diabetic neuropathy. Changes in both afferent and efferent innervation to the bladder in conjunction with alterations in conduction velocity o f afferent fibers have

been described in diabetic rats (Steers et al 1994). The sensory nerves are thought to be affected first, leading to impaired sensation o f bladder filling (Smith et al 1917, Buck et al 1974, Ellenberg et al 1980). Alteration in bladder emptying in DM is thought to be due to abnormalities in the conduction o f afferent Aô-fibers, which are involved in both the spinal and supraspinal micturition reflexes (Steers et al 1990). These abnormalities correlate well with the structural changes in the ganglia and neurons innervating the bladder (Medori et al 1988). Interestingly, indirect evidence has also come from studies using vasodilator agents, indicating that diabetic neuropathy could also result from a deficit in the blood flow to nerves. Thus, in diabetic rats, vasodilator agents, such as angiotensin II and ET receptor antagonists, corrected defects in nerve conduction velocity (Cameron et al 1996). Furthermore, the NO donor, isosorbide dinitrate also restored conduction and nerve blood flow abnormalities in DM (Cameron et al 1995). These findings suggest that alterations in ET and NO bioactivity may contribute to the pathogenesis o f diabetic neuropathy leading to bladder dysfunction.

1.6.2 Autonomic receptor andfunctional changes

Alterations in the expression and function o f various type o f receptors has been described in both clinical (Faerman et al 1973, Buck et al 1976) and experimental DM (Lincoln et al 1984,b Kolta et al 1985, Longhurst et al 1986, Moss et al 1987, Luheshi et al 1990b, Morita et al 1991, Nakamura et al 1992). For example, sensitivity to purinergic agonists is high in 8-week diabetic animals and markedly reduced in 16-week diabetic animals

(Moss et al 1987). However, the responsiveness o f diabetic smooth muscle strips to cholinergic and adrenergic agonists may increase (Kolta et al 1985), decrease (Longhurst

et al 1986) or remains unchanged (Lincoln 1984a) compared to controls. Potentiation of the cholinergic motor transmission is thought to be secondary to enhanced release and activity o f ACh in the diabetic detrusor (Luheshi et al 1990a, Dail et al 1977, Lincoln et al 1984a).

1.6.3 Structural changes o f the bladder smooth muscle.

Increased urine output in DM causes bladder wall distension. The bladder responds to distension with a rapid and substantial increase in bladder mass (Lincoln et al 1984b, Uvelius et al 1986, Eika et al 1993) with concomitant alterations in the smooth muscle contractile responses to various stimuli (Longhurst et al 1986, Santicioli et al 1987, Latifpour et al 1989). In the rat DM model, both hyperplasia (increased in cell number as a result o f increased cell division) as well as hypertrophy (increase in cell size) is evident in the urinary bladder. An increase in DNA synthesis and [^H]-thymidine uptake (Levin et al 1994) has been clearly demonstated in the DM rat bladder. However, there are no histological studies available that describe the structural changes in the diabetic human and rabbit bladder.

Studies o f growth factor expression during the early period o f bladder hypertrophy have also proven to be interesting. These have identified changes in the expression o f several growth factors (e.g. heat-shock protein-70, basic fibroblast growth factor (bFGF) and transforming growth factor-beta (TGF-p), which may play a significant role in the development o f detrusor hypertrophy in response to bladder distension (Levin et al 1994). Apoptosis and restoration o f growth factor expression

accompany regression o f bladder hypertrophy and hyperplasia following the reversal of bladder distension (Levin et al 1994). These findings imply that bladder hypertrophy/hyperplasia and apoptosis are directly opposing processes that are controlled by different effects on the gene expression o f growth factors (Santarosa et al 1994). In this context, several reports indicate that ET stimulates mitogenesis (Muldoon et al 1989, 1990, Takuwa et al 1989, Walden et al 1998) and that NO enhances apoptosis and inhibits cellular proliferation (Guh et al 1998). Indeed ET-1 is also thought to enhance the mitogenic effects o f several growth factors. In addition, the increased production o f PGL, by the STZ-induced diabetic rats is also associated with a significant increase in bladder weights.

Several animal models have been used to investigate the pathophysiology of diabetic bladder dysfunction. These have included dogs (Shishito et al 1964), Chinese hamsters (Dail et al 1977), streptozotocin-(Lincoln et al 1984a,b) or alloxan-induced diabetic rats (Uvelius et al 1986, Paro et al 1990), spontaneous diabetic rats (Marliss et al 1982) and alloxan-induced diabetic rabbits (Gupta et al 1996). Although, the rat has been the most widely used animal model for the study o f the effect o f DM on bladder function, it suffers from severe metabolic effects o f starvation leading to marked growth retardation and loss o f weight. Furthermore, in the rat model there is marked hypertrophy o f the bladder, which at two months after induction o f DM is three to five times greater in diabetic animals compared to age-matched controls (Lincoln et al 1984b, Miller et al 1994). Since, clinical DM is not associated with such extremes the validity o f the rat model for the investigation o f the effects o f DM on the urinary bladder is debatable. In the rabbit model, however, there is only a modest decrease in body weight and minimal

hypertrophy o f the urinary bladder. Furthermore, unlike the rat, which suffers from tissue availability, the bladder and urethra o f the rabbit are large enough to carry out both receptor and functional analysis in the same group o f animals. Finally, due to the similarity o f distribution and function o f most autonomic receptors in the rabbit and human, we selected the rabbit model to investigate the role o f ET, NO and PGs in experimental DM.

CHAPTER 2