The In Vitro Contractile Response of Canine Pregnant Myometrium to Oxytocin and Denaverine Hydrochloride

Abstract

In pregnant bitches, the response to oxytocin and denaverine hydrochloride in dystocia management is usually poor. To better understand the effect of both drugs on myometrial contractility, the circular and longitudinal muscle layers were examined in an organ bath. For each layer, three myometrial strips were stimulated twice, each with one of three oxytocin concentrations. The effect of denaverine hydrochloride was studied once in direct combination with oxytocin and alone with subsequent oxytocin administration. Contractions were recorded and evaluated for average amplitude, mean force, area under the curve (AUC), and frequency. Effects of different treatments were analyzed and compared within and between layers. In the circular layer, oxytocin significantly increased amplitude and mean force compared to untreated controls regardless of stimulation cycles or concentrations. In both layers, high oxytocin concentrations caused tonic contractions, while the lowest concentration created regular rhythmic contractions. Longitudinal layer tissue responded to oxytocin with a significantly decreased contractility when stimulated twice, presumably a sign of desensitization. Denaverine hydrochloride neither affected oxytocin induced contractions nor showed a priming effect to subsequent oxytocin. Thus, no benefit of denaverine hydrochloride on myometrial contractility was found in the organ bath. Our results suggest a better efficiency of low-dose oxytocin in canine dystocia management.

Keywords: canine; contractility; denaverine hydrochloride; dystocia; myometrium; oxytocin; parturition.

Figure 3

(a) Amplitude, (b) mean force, (c) AUC, and (d) frequency of contractions of circular myometrial strips stimulated with 1 nM, 10 nM, and 100 nM oxytocin. Responses following stimulation differed significantly from respective untreated controls (## p ≤ 0.01, ### p ≤ 0.001). The results are presented as mean ± standard deviation. Bars with asterisks differ significantly (* p ≤ 0.05; ** p ≤ 0.01).

Figure 4

(a) Amplitude, (b) mean force, (c) AUC, and (d) frequency of contractions of longitudinal myometrial strips stimulated with 1 nM, 10 nM, and 100 nM oxytocin. Responses following stimulation differed significantly from untreated controls (# p ≤0.05, ## p ≤ 0.01). The results are presented as mean ± standard deviation. Bars with asterisks differ significantly (* p ≤ 0.05; ** p ≤ 0.01; *** p ≤ 0.001).

Figure 5

Comparison of the circular and longitudinal myometrial layer responding to 1 nM, 10 nM, and 100 nM oxytocin during the first (1st) and second (2nd) stimulation with oxytocin. (a) Amplitude, (b) mean force, (c) AUC, and (d) frequency of contractions were analyzed and presented as mean ± standard deviation. Bars with asterisks differ significantly (** p ≤ 0.01; *** p ≤ 0.001).

Figure 1

An example of typical contraction patterns of isolated canine myometrium. Four strips of the longitudinal (blue) and circular (red) layer were mounted in the organ bath recording spontaneous contractions (TF1). (TF2) Strips were stimulated with different concentrations of oxytocin (line 2/6: 100 nM; line 3/7: 10 nM; line 4/8: 1 nM) and afterwards washed with buffer (TF3). This stimulation cycle was repeated (TF4).

Figure 2

An example of an experiment investigating the effect of denaverine hydrochloride on canine pregnant myometrium in vitro. Strips of the longitudinal (blue) and circular (red) layer were stimulated with 1 nM oxytocin, followed one min later by the addition of either 1.97 nM denaverine (TF2; line 1/5) or 2.97 nM denaverine (TF2; line 2/6). One strip per layer was exposed to only 1 nM oxytocin (TF2; line 3/7) or 2.97 nM denaverine (TF2; line 4/8). Strips treated with denaverine only in TF2 were stimulated with oxytocin in the second stimulation cycle (TF4; line 4/8).

Figure 6

Analysis of up to 10 contractions (a) during the first (1st) and second (2nd) stimulation with oxytocin gave slightly different AUC results than considering only the initial response to oxytocin (b). The results are presented as mean ± standard deviation. Bars with asterisks differ significantly (** p ≤ 0.01).

Figure 7

Investigation of the combined effect of oxytocin and denaverine and oxytocin only on the amplitude (a,e), mean force (b,f), AUC (c,g), and frequency (d,h) of contractions for the two myometrial layers, respectively (red = circular; blue = longitudinal). + indicates what has been applied to the samples. No statistically significant differences were found. The values are presented as mean ± standard deviation.

Figure 8

Comparison of the response of the circular (red) and longitudinal (blue) myometrial layer to oxytocin alone (no pattern) and after prior priming with 2.97 nM denaverine (black stripes). No differences between the treatments were found for the amplitude (a,e), mean force (b,f), AUC (c,g), and frequency (d,h) of contractions. Results are presented as mean ± standard deviation.