I have tried using DMT wire myograph set-up following the instruction manual but this was devised for mesenteric arteries. Physiologically pulmonary veins would be under a greater degree less basal tone and are more fragile so this is where my issue arises. Any help or advice on the matter would be much appreciated.
Amy, we routinely use pulmonary arteries and small bronchioles on DMT myographs, where the transluminal pressure (i.e. stretch) is much less than for mesenteric or other systemic arteries. Note though that optimum resting tension is not necessarily the same as physiological. As a guide, the resting tension should be that required to give approximately a transluminal pressure equivalent to normal pulmonary venous pressure (~8-10 mmHg). This will depend on the vessel size (.i.d.), and can be calculated from the modified Laplace equation as shown in the DMT myograph instructions (or if it is an automated version, you just put the required transluminal pressure into the controller. Alternatively you can use a rather more laborious method (which we have to use for airways), where you repetitively stretch, allow to recover, then challenge with 80mM [K+] (isoosmotic substitution for NaCl). Optimal stretch is when you get the largest response to high [K+]. You have to be careful not to over-stretch however. To do this properly, you should incrementally increase the jaw gap from touching, then do the same thing in reverse and graph the results - the original Mulvany method. We originally published the protocols for use of the DMT myograph with pulmonary arteries in (I think) Leach RM, Cameron IR, Twort CH and Ward JP.
Q J Exp Physiol. 1989 Nov;74(6):947-50. This could be easily adapted for the lower pressure veins.
Thank you for your comprehensive advice Jeremy. It is very much appreciated and will help with my future studies.
Apologies Jeremy but can I confirm that I would use the equation Pi = Wall tension/(internal circumference/(2π))? As I know the Pi already I do not have to undertake the full normalisation procedure which stretches the vessels to a transmural pressure of 100mmHg and would likely damage pulmonary veins significantly? A final question I have about this procedure: for internal circumference calculation, would I do this at a point where the myograph jaws are almost touching or what would be the best place to take this measurement? (I am aware this would strongly influence the final output from the equation.) Also, as far as I can see I would use vessel length in the calculation (for wall tension - force divided by vessel length) but not the external diameter? Am I correct in saying this?
Kind regards
Amy
Yes, you use that equation - wall tension is per mm vessel length (which is why you measure the length). You should do the full normalisation procedure, especially because few people have used pulmonary veins on a myograph and you should characterise the preparation. However, you do NOT go up to 100mmHg - that is only for systemic arteries. I would go up to a max of about 15 mmHg first to see whether this causes overstretch; veins are very compliant, so easy to overstretch. Basically, the process is to stretch slightly from jaws touching, and once the tension has stabilised you calculate the new effective transmural pressure (Pi) from the tension and the calculated internal circumference. Then you do this over and again until the Pi is at your preferred maximum. Then reverse the process. If you then plot all the jaw widths against Pi you can select the jaw width that provides the Pi you want (say 10mmHg). This is the same process as the normal normalisation procedure, but with a much reduced maximum Pi.
The final internal circumference is what you calculate when the vessel is set to the Pi you want.
I would try with a few test vessels first to get an idea of how they react. We tend to challenge with high K PSS after each stretch as well, as that gives you the active contractile response. That should initially increase, but will start to decrease if you overstretch.
Hope this is helpful
Jeremy W
Thank you very much for your clear and detailed explanation of the procedure - I will aim to characterise this normalisation in the near future.
Amy
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