Paired pulse analysis of ATP and noradrenaline release from sympathetic nerves of rat tail artery and mouse vas deferens: effects of K+ channel blockers

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Abstract
The paired pulse stimulus paradigm – two pulses of equal strength delivered at variable interpulse intervals – was used to study the release of ATP and noradrenaline (NA) from post ganglionic sympathetic nerves of rat tail artery and mouse vas deferens. Excitatory junction currents (EJCs) were used to measure the release of ATP, and differential pulse amperometry to measure that of NA. At interpulse intervals of 0.1–1 s paired pulse stimulation caused an increase in the size of the second EJC, both in rat tail artery and mouse vas deferens. As the interpulse interval was increased to 10 s or more, the two EJCs became of equal size. In both preparations the K+ channel blockers tetraethylammonium (TEA, 20 mM) and 4‐aminopyridine (4‐AP, 1 mM) prolonged the duration of the nerve terminal spike and greatly amplified the first EJC of the pair. In the presence of TEA and 4‐AP in rat tail artery paired pulse stimulation caused a dramatic depression of the second EJC without markedly affecting the nerve terminal spike. The depression of the second EJC decreased with increasing interpulse intervals, and also when external Ca2+ was reduced to 0.2 mM. In mouse vas deferens, TEA and 4‐AP caused only a modest depression of the second EJC. In rat tail artery in the presence of TEA and 4‐AP paired pulse stimulation caused a depression of the NA oxidation current evoked by the second pulse, which was similar in magnitude and time course to that of the EJC. Similar TEA and 4‐AP induced depression of the second pulse response was also observed when the purinergic and noradrenergic components of the contractile response were investigated. The results show that in rat tail artery K+ channel blockers cause a dramatic paired pulse depression of the release of ATP and NA. The similarity in the depression of the EJC, the NA oxidation current, and the purinergic and noradrenergic components of the contractile response is compatible with the hypothesis that ATP and NA are released in parallel from the same neuronal sources.

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