Transient Receptor Potential Function in Bladder from Control and Streptozotocin Treated Rats
Diabetic cystopathy is a chronic and common complication of diabetes with a classical triad of symptoms; decreased bladder sensation, increased bladder capacity and impaired detrusor muscle contractility (Hunter and Moore, 2003). In animal models of diabetes such as streptozotocin-induced diabetes in the rat, abnormalities of bladder function have been reported (Longhurst and Belis, 1986). The prototypic TRPV channel, TRPV1, is activated by capsaicin, which has been shown to cause contraction of the rat bladder (Saitoh et al., 2007), and this is reduced in STZ-diabetic rat bladder (Pinna et al., 1994). Therefore we hypothesize that TRPV1 function will be reduced in the diabetic bladder. The aim of this study are the following: Firstly, to investigate the effect of the streptozotocin (STZ) model of diabetes on a range of TRP channel functions in the urinary bladder smooth muscle preparation using TRP channel agonists and antagonists and to study the neurotransmitters involved in the contractile or relaxant responses. Some studies were also performed on colon tissues. Secondly, to explore the involvement of cholesterol modudation in TRP channel signalling. Thirdly, to study the change in TRP channel response with time following the treatment with streptozotocin. The results showed that the contractile responses to the TRPV1 agonist capsaicin, TRPV4 agonist 4-α-PDD, and TRPA1 agonist allyl isothiocyanate were significantly reduced in diabetic bladder. The selective TRPV1 antagonist, SB-366791, inhibited the contractile responses to capsaicin confirming the involvement of TRPV1 channels. The effect of diabetes is unlikely to be at the level of contractile machinery since the contractile responses to muscarinic receptor agonist carbachol were not significantly reduced in diabetic tissues. It is reported for the first time that the combination of neurokinin 1 and 2 antagonists GR-205171 and SB-207164 inhibited the contractile responses to capsaicin suggesting that a neurokinin may be the neurotransmitter involved in the capsaicin responses. In addition, the reduction of the responses to capsaicin in STZ-induced diabetic tissues occurred not only in urinary bladder but also in colon. Cholesterol-PEG significantly lowered the maximal contractile responses to capsaicin of rat bladder strips. Methyl-β-cyclodextrin, α-cyclodextrin and β-cyclodextrin at the same concentrations enhanced the contractile responses to capsaicin in the control and diabetic rat bladder strips. These effects of cyclodextrin are specific to capsaicin activated contractions and not seen with TRPA1 activation, suggesting that the effects are not mediated downstream of channel activation. Since α-cyclodextrin does not sequester cholesterol, the enhanced responses to cyclodextrins may not be due to the cholesterol modulations. Instead, theses novel findings may possibly occur by changing the local membrane lipid environment of the TRPV1 channel. As early as 36 hours after induction of diabetes by STZ, the contractile responses to capsaicin were significantly reduced in comparison to those of the controls and this reduction persisted until the eight weeks time point. In contrast, responses to the TRPA1 agonist allyl isothiocyanate were not affected at early time points but were reduced one week after STZ treatment. This detailed time course analysis suggests that there are novel mechanisms of modulation of the TRPV1 channels in this STZ model. In conclusion, in the rat urinary bladder or colon preparations, diabetes mellitus using STZ animal model caused 1) the impairment of a number of TRP channel subfamily functions, TRPV1, TRPV4 and TRPA1 but not TRPM8. The combination of NK1 and NK2 antagonists significantly inhibited the responses to capsaicin. This may suggest the involvement of neurokinin in postsynaptic transmission in rat bladder following the activation of TRPV1 channel, 2) the impairment caused by STZ-induced diabetes occurred very early (within 36 hours after diabetes induction) in TRPV1 channel but not TRPA1 channel. There are specific early effects of STZ treatment on TRPV1 channel function at a time when other afferent nerve terminal channels (TRPA1) are functioning normally, suggesting that early onset of dysfunction in TRPV1 signalling may not merely be the consequence of nerve damage, 3) the mechanism of this impairment may not be the effect of neuropathy on neurotransmitter release or nerve damage. Improving the responsiveness of nerves of bladder in diabetic patients might be of therapeutic benefit. The present studies suggest that it is possible to enhance function using indirect modulators such as bradykinin which potentiated the TRPV1 channel function in diabetic rat bladders.