Apparent depletion of NE stores after repetitive stimulation of cat colon in presence of phenoxybenzamine

The action of extracellular cations on the release of the sympathetic transmitter from peripheral nerves

1. The noradrenaline stores in the sympathetic nerve endings of the cat colon were labelled in vivo with (+/-)-[7-(3)H]noradrenaline (500 muc, 11.3 mug) injected 3 hr before killing.2. The colon was removed and immersed in an organ bath containing Krebs solution. The vascular bed was perfused from the inferior mesenteric artery to the colic vein.3. The effects of Ca(2+), Ba(2+) and Mg(2+) on the output of [(3)H]noradrenaline and [(3)H]metabolites in the venous effluent were measured before and after electrical stimulation of the post-ganglionic inferior mesenteric nerves at 10 impulses/sec.4. Nerve stimulation increased the efflux of [(3)H]noradrenaline when the perfusion fluid contained Ca(2+). Variations in Ca(2+) concentration (1.5-10 mM) did not affect this response.5. Removal of Ca(2+) from the fluid passing through the vascular bed (and therefore from the region of the sympathetic nerve terminals), abolished the output of [(3)H]noradrenaline in response to nerve stimulation. There was no change when phenoxybenzamine was added to prevent the binding of transmitter on to post-synaptic receptors on the effector organ.6. The output of transmitter was not changed when Ca(2+) was present in the blood vessels of the colon even though it was removed from the solution in contact with the remainder of the tissue.7. Nerve stimulation released [(3)H]noradrenaline when Ba(2+) was used as a substitute for Ca(2+); Mg(2+) was not an effective substitute for Ca(2+) as then nerve stimulation did not increase the output of radioactive noradrenaline or metabolites.8. Ba(2+) also increased the resting output of [(3)H]noradrenaline in the absence of nerve stimulation. Addition of Ca(2+) or Mg(2+) did not change this spontaneous release but it was augmented by removal of Ca(2+).9. It is concluded that Ca(2+) is essential for release of the sympathetic transmitter by nerve stimulation but not for the spontaneous output that occurs in the absence of nervous activity. The site of action of Ca(2+) is considered to be the terminals of adrenergic fibres.

Negative feed-back regulation of noradrenaline release by nerve stimulation in the perfused cat's spleen: differences in potency of phenoxybenzamine in blocking the pre- and post-synaptic adrenergic receptors

1. The effects of low concentrations of phenoxybenzamine (8.8 x 10(-10) to 2.9 x 10(-7)M) on responses and on noradrenaline overflow elicited by nerve stimulation were studied in the perfused cat's spleen.2. In the presence of 8.8 x 10(-10)M or 2.9 x 10(-9)M phenoxybenzamine there was a significant reduction in responses to nerve stimulation while the overflow of the transmitter did not increase at the two frequencies of stimulation employed: 5 and 30 Hz.3. In the presence of 2.9 x 10(-8)M or 2.9 x 10(-7)M phenoxybenzamine the responses to nerve stimulation were practically abolished and a significant increase in transmitter overflow was obtained at both frequencies of stimulation. The drug was more effective in increasing transmitter overflow at 5 Hz when compared with 30 Hz.4. The higher effectiveness of phenoxybenzamine in blocking the post-synaptic alpha-receptor when compared with the blockade of the presynaptic alpha-receptor that regulates transmitter release is compatible with the view that these two receptors are not identical.5. A second alternative for the difference in effectiveness of phenoxybenzamine is that both types of alpha receptors are identical, but the spare receptor capacity for the presynaptic adrenergic receptors is higher than that of the post-synaptic receptors.