**Author details**

134 Neuroendocrinology and Behavior

et al., 1996; Chan et al., 1996; Keskil et al., 1999).

parameter.

in the same concentration as Ang II induced tonic contractions with amplitude of 6.61 ± 0.39 g (n = 8) and an integral force of muscle contraction of 7245.00 ±901.00 gs. The duration of the AVP-induced responses was several times greater than those of Ang II and the recording of AVP-mediated contractions was stopped on the 30th minute without achievement of Tchr

Our experiment confirmed the contractile effect of these two peptides on the myometrium, which is in accordance with the results of other authors working on the same issues (Anouar

The contractions induced by both peptides have similar amplitudes, but they are with different duration and characteristics. The registered AVP - provoked uterine responses were found to have a sustained oscillating character Figure 6). When analyzed by mathematical modeling such contractions were recognized as underdamped process - the system tries to establish a stable level different from the baseline (Yankov, 2009). The differences in the developed contractions may be due to split of the classical or inclusion of additional transduction pathway for each of the studied peptides. Both of them have several main groups of receptors. The receptors for Ang II are AT1 and AT2 (De Gasparo et al.,

To establish the importance of these receptors for the uterine muscle contraction will be the

First – the constrictor effect of Ang II is associated with AT1 receptors, but the uterus is one of the few organs with a. uterina inferior where AT2 receptors are predominant (Keskil et al., 1999). AT2 receptors are mainly regarded to oppose the effects of AT1 and cause dilation, blood pressure reduction, nitric oxide production (Hannan et al., 2003). Perhaps the significantly shorter phase of contraction and relaxation was due to their activation under the influence of Ang II in the uterus. Second – the constrictor effect of AVP is realized by V1a receptors which are found in uterine arteries. With regard to the contractile response of the myometrium, however, there are statements that the resulting contraction from the AVP influence is due to activation of other receptors, different from the mentioned above (Anouar et al., 1996). Some authors go even further and argue that AVP accomplish its effect on uterine musculature by OT receptors, which have big similarity with V1 receptors (Chan et al., 1996).

Considering that both peptides are released from supraoptic nuclei in the hypothalamus and that they have a powerful contractile effect on the smooth muscle, it is appropriate to

subject of our next experiments. However, several interesting facts immerge:

2000), while the receptors for AVP are V1a, V1b and V2 (Petersen, 2006).

**Figure 6.** Original record of vasopressin-induced uterine contraction.

Anna Tolekova, Petya Hadzhibozheva, Tsvetelin Georgiev, Stanislava Mihailova and Galina Ilieva *Department of Physiology, Pathophysiology and Pharmacology, Medical Faculty, Trakia University, Stara Zagora, Bulgaria* 

Maya Gulubova *Department of General and Clinical Pathology, Medical Faculty, Trakia University, Stara Zagora, Bulgaria* 

Eleonora Leventieva-Necheva, Kiril Milenov and Reni Kalfin *Institute of Neurobiology, Bulgarian Academy of Sciences, Sofia, Bulgaria* 
