Studies on the perfused rabbit ear artery preparation showed that
withdrawal of Mg 2+ from extracellular fluid potentiated the responses
to histamine and ATP but not to catecholamines. Similar results were
obtained in [2xCa2+] Krebs solution. Increases in [Mg 2+] decreased
responses to the three agonists to a similar extent. In subsequent
experiments attempts were made to alter the availability of calcium for
contraction induced by these agonists either by changing the [Ca 2+]
of the Krebs solution or by using Ca 2+ influx inhibitors, ouabain and
ryanodine. The effects of these agonists were compared to those
observed when Mg2+ was altered. In general, the results obtained
in perfused rabbit ear artery supported the hypothesis that changes in
extracellular [Mg2+] affect the availability of calcium for contraction
but were not consistent with the suggestion that Mg2+ alters Ca2+ influx.
In a second type of preparation tension responses of superfused
rings of ear artery were studied. Responses to changes in extracellular
[Ca2+] and[ Mg2+] were found to differ slightly from those
obtained in the perfused artery. A simultaneously perfused and
superfused arterial preparation showed that responses to changes in
[ Mg2+] and[Ca2+] were different if the agonist was administered to
the adventitial surface of the vessel rather than via the intimal
surface.
The effects of alterations in extracellular [Mg 2+] were studied in mesenteric arteries from weight matched normotensive and spontaneously
hypertensive rats (SHR). No differences in response to NA or
ATP when extracellular [Mg 2+ ] was either increased or reduced were
observed in the SHR compared to the normotensive animal. However,
a difference in calcium dependence was demonstrated between the two
types of vessels to NA. In contrast to mesenteric arteries, experiments
on aortae from normotensive rats and SHR showed no differences
in the calcium dependence of NA responses between normotensive and
SHR vessels, whereas,
[4xMg2+ ] Krebs solution reduced the responses
of normotensive aorta to NA more than SHR. These results in the
rat were not consistent with the hypothesis that alteration in [Mg 2+]
can be explained in terms of altered calcium availability. Attempts
to increase intracellular cyclic AMP with theophylline showed that
the response to ED50 NA in both mesenteric arteries and aortae from
normotensive were reduced more than SHR.
It is concluded that the effect of changes in extracellular [Mg2+]
on the reactivity of vascular muscle varies depending on the type of
vessel and species of animal from which the vessel is taken. In
addition when all the experimental results are considered, it is not
possible to explain all the actions of altered [ Mg2+ ] simply in
terms of changed calcium availability.
| Identifer | oai:union.ndltd.org:BRADFORD/oai:bradscholars.brad.ac.uk:10454/4001 |
| Date | January 1982 |
| Creators | Asmawi, Mohd. Z. |
| Contributors | McCurrie, Janice R., Irving, G. |
| Publisher | University of Bradford, Postgraduate School of Studies in Pharmacology |
| Source Sets | Bradford Scholars |
| Language | English |
| Detected Language | English |
| Type | Thesis, doctoral, PhD |
| Rights | <a rel="license" href="http://creativecommons.org/licenses/by-nc-nd/3.0/"><img alt="Creative Commons License" style="border-width:0" src="http://i.creativecommons.org/l/by-nc-nd/3.0/88x31.png" /></a><br />The University of Bradford theses are licenced under a <a rel="license" href="http://creativecommons.org/licenses/by-nc-nd/3.0/">Creative Commons Licence</a>. |
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