12
WIRE MYOGRAPH SYSTEM - 620M
- USER GUIDE
Figure 3.9 Illustration of the exponential
curve fitting and determination of IC
100
Figure 3.8 Illustration of the stepwise
normalization procedure
The aim of the normalization procedure is to stretch the segment to a so-called normalized internal circumference (IC
1
): defined
as a set fraction of the internal circumference (IC
100
) that a fully relaxed segment would have at a specified transmural pressure.
For small rat arteries the target transmural pressure is typically 100 mmHg = 13.3 kPa.
3.2.1 Principles of the normalization procedure
In practice the normalization is performed by distending the segment stepwise and measuring sets of micrometer and force read-
ings (figure 3.8). These data are converted into values of internal circumference (μm) and wall tension T (mN/mm) respectively.
Plotting wall tension against internal circumference reveals an exponential curve and by applying the isobar curve corresponding
to 100 mmHg, IC
100
is calculated from the point of intersection using the Laplace relation (figure 3.9). IC
1
is calculated from IC
100
by multiplying a factor giving an internal circumference at which the active force production as well as the sensitivity to agonists
of the segment is maximal. For rat mesenteric arteries the factor is 0.9 but both this factor as well as the transmural pressure has
to be optimized for each particular segment. The normalized internal diameter is calculated by dividing IC
1
with .
3.3 Standard start
The purpose of performing a standard start is to:
1. Re-activate the mechanical and functional properties of the vessel segment.
2. Check that responses to different types of stimuli are normal in appearance and thereby ensuring that the functionality of
the vessel segment has not been damaged during the dissection or mounting procedures.
3. Ensure that the tension development gives an effective active pressure that is above the chosen accepted value (usually
13.3 kPa = 100 mmHg).
The standard start is performed after the vessel segment has been heated, equilibrated and normalized. The present procedure
is suitable for rat mesenteric arteries. Another procedure may be needed for other animal species and tissue or vessel types.
3.3.1 Principles of the standard start procedure
The standard start procedure consists of a series of five stimuli and washout periods. The first two stimuli are performed using
a mixture of KPSS and 10 μM noradrenaline to give a maximum contractile response. The third stimulus is performed using a
mixture of PSS and 10 μM noradrenaline to give a maximum pure agonist mediated (α-adrenoceptor) contraction. The fourth
stimulus is performed using KPSS to give a depolarising contractile response (this stimulus also includes a component from neu-
rally released noradrenaline). The final stimulus is performed using a mixture of PSS and 10 μM noradrenaline. All solutions are
preheated to 37
o
C and aerated with a mixture of 95% O
2
and 5% CO
2
before use. Instructions for making the necessary solutions
are described in appendix 1.
CHAPTER 3
