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THE ROLE OF CHLORIDE IN NORADRENALINE-STIMULATED RAT MESENTERIC SMALL ARTERY VASOMOTION

Donna Marie Briggs Bødtkjer

Summary

Vasomotion is a vascular phenomenon that was first described in the mid 19th century. Vasomotion is a regular oscillation in vascular tone; a sinusoidal pattern of vasoconstriction and vasodilatation that is not related to the pressure waves generated by the heart. It is described as a local autoregulation mechanism and is especially prevalent in the small resistance arteries that contribute to blood pressure regulation. Vasomotion is a much conserved vascular phenomenon, seen across a wide number of species, making it ideal for study in comparative biological systems. Based upon previous experimental evidence, a model of vasomotion was proposed for rat mesenteric resistance arteries. The work of this thesis has addressed to what extent noradrenaline-stimulated vasomotion in these arteries depends upon chloride and whether the cGMP-dependent calcium activated chloride conductance present in the membrane of vascular smooth muscle cells (VSMCs) is critical for vasomotion.  

Studies performed with 36Cl in small mesenteric resistance arteries from male Wistar rats, confirmed that intracellular chloride is maintained at a high level enabling chloride to act as a depolarising current in these arteries. Isometric force measurements made from segments of isolated mesenteric arteries enabled the characterisation of vasoconstriction and vasomotion in response to noradrenaline (NA) application. Vasomotion was found to be very sensitive to the presence of normal levels of extracellular chloride. Chloride removal (and substitution with aspartate) resulted in a significant inhibition of vasomotion without otherwise altering vascular reactivity to NA. Furthermore, studies performed with another anion, thiocyanate (SCN), demonstrated that an ‘anion block’ of chloride channels can also inhibit vasomotion. Several different experimental approaches, including patch-clamp electrophysiology, showed that the abolition of vasomotion was most likely due to the interference of SCNwith the chloride conductance, leading to a disruption of Ca2+i synchronisation and not with other known steps in the vasomotion model. Further studies employed the chloride channel antagonist DIDS (4,4'-diisothiocyanato-stilbene-2,2'-disulphonic acid) to confirm the chloride-dependency of vasomotion. Unexpectedly, this did not result in data directly comparable to the chloride substitution studies. In the presence of 1 mM DIDS, vasomotion persisted and became cGMP-independent, that is, the presence of an endothelium was not necessary in order for oscillations to be stimulated by NA. The applied concentration of DIDS significantly blocks all characterised chloride currents present in VSMCs, but also has other effects including an increase in VSMC membrane resistance. This might allow another depolarising current to maintain vasomotion in the presence of DIDS. A candidate for this channel could be a canonical transient receptor potential channel (TRPC) as members of this family have been identified as being stimulated by NA. 

In summary, we conclude that chloride is important for the NA-stimulated vasomotion in rat mesenteric small arteries as described by the published model. The chloride current responsible for this appears to be the cGMP-dependent calcium-activated conductance. When DIDS is present, and chloride conductances are blocked, a new oscillation can be elicited that is cGMP (endothelium) independent.