While several neurotransmitters have been shown to elicit detrusor contractions, less is known about direct functional effects of hormones. The steroid hormone progesterone, present in both males and females, has been shown to induce bladder smooth muscle relaxation and increase bladder capacity and compliance [1,2]. However, less is known about its “anti-contractile” properties to major parasympathetic transmitters, and the time course thereof. Thus, the aims of the present study were to investigate whether progesterone causes a sustained functional relaxation able to counteract muscarinic and purinergic contractions and, if so, what the duration of this response is after removal of the hormone.

Male Sprague-Dawley rats (n=7) were euthanized and the urinary bladders harvested in accordance with the ethics approval by the local ethics committee. Two to three full thickness bladder strips were excised from each bladder, from above the trigone and proximal to the ureters, and were mounted in organ baths (20mL, Biopac systems Inc., Goleta, CA; MP35, SS63L force transducer). Functional responses were recorded and analyzed using BSL Analysis 4.1 and GraphPad Prism 10.4.1 software.

Responses to agonists were recorded at baseline, after which progesterone (5x10-4 M) was added and let to equilibrate for 10 min before washed away. Then, agonists were once again administered after another 10 min. For time course experiments, methacholine (MeCh) and ATP was added in sequence every 20 min. 

MeCh or ATP were dissolved in milli-Q water and administered cumulatively in volumes of 100 uL, whereas progesterone was dissolved in DMSO and added in a volume of 500 uL. A separate investigation of any impact of the vehicle on contractility was performed.

Statistical significance was determined by 2-way ANOVA, computed using GraphPad Prism.

The vehicle (DMSO) did not change the responses to MeCh or ATP. However, progesterone (5x10-4 M) substantially reduced contractions to MeCh (10-5 - 10-3 M) by 68.1 % and ATP (10-3 and 5x10-3 M) by 88.2 % at the highest concentrations, respectively (Fig. 1).

The progesterone evoked reduction in contraction was further shown to be remarkably sustained. After 90 minutes, the contractility of the bladder strips to 10-3 M MeCh was recovered, i.e. no longer statistically different from baseline (p<0.05, Fig 2a). Lower concentrations of MeCh (10-5 M) did however not produce an equally convincing recovery.

For ATP the pattern was similar, and contractions to 5x10-3 M ATP initially dropped by 84.2 % but were eventually restored, albeit the purinergic responses were smaller and more variable than the cholinergic (Fig 2b). Lower concentrations of ATP (10-3 M) failed to yield robust responses.

Progesterone causes not only a relaxation, but an anti-contractile functional response with enough force to counteract parasympathetic contractions. Using rather high, pharmacological, concentrations of MeCh and ATP, the anti-contractile effects of progesterone can eventually be overcome. However, the relaxatory effects are sustained and last for more than an hour after removal of the hormone. This further highlights the critical role of endocrine signaling in maintaining healthy bladder function.

Progesterone potently counteracts parasympathetic contractions in the rat urinary bladder. Further research should be focused on the exact mechanisms behind these sustained responses, and whether changes occur in bladder diseases and/or between the sexes.

V. S. Fernandes et al., Underlying mechanisms involved in progesterone-induced relaxation to the pig bladder neck. Eur J Pharmacol 723, 246-252 (2014)
S. Celayir, Effects of different sex hormones on male rabbit urodynamics: an experimental study. Horm Res 60, 215-220 (2003)