Model of counter-current transfer from vagina to urethra in postmenopausal women

BACKGROUND: Low-dose vaginal oestrogens are effective in treating post-menopausal urogenital atrophy without inducing endometrial proliferation. We aimed to assess whether this dichotomic effect could be the result of a preferential vagina-to-urethra transfer via a counter-current transfer of oestrogens from vagina to the arterial blood supplying the urethra. Due to the impossibility of obtaining blood samples from urethral arteries, and since the nature of counter-current exchange of substances is similar to the transfer of heat, we investigated cold transfer throughout the anterior vaginal wall to the vesical trigone and urethra. METHODS: Plastic tubes filled with cold saline were inserted into the vagina of 30 menopausal women. Temperatures were recorded at the vesical trigone and at three different urethral sites. Comparisons were performed 2 and 4.5 min after starting of cooling, and 4.5 min after removal of tubes. RESULTS: The urethra cooled significantly more than the bladder. Urethral cooling was not homogeneous; distal sites of the urethra cooled significantly more than proximal site and bladder despite a larger distance to the vaginal cooling device. CONCLUSIONS: Distribution of cold from the vagina to the urethra is not the result of simple diffusion and mechanisms of preferential distribution may exist from the vagina to the middle and low part of the urethra.