Contraction of the anterior prostate is required for the initiation of micturition

WHAT'S KNOWN ON THE SUBJECT? AND WHAT DOES THE STUDY ADD?: In a recent rtMRI study, we were able to show that, during initiation of voiding, there was both funnelling of the bladder neck and simultaneous contraction of the ventral prostate. We presumed that the vertical contraction of the ventral prostate contributes to the initiation of successful micturition. The question remained as to whether this shortening of the ventral prostate is predominantly caused by contractile elements in the organ itself, or by surrounding contractile elements of the pelvic floor. In our study we provide insight in to anatomical changes, and biometric and functional analysis of the prostate during micturition. A sagittal contraction of the ventral prostate and the longitudinal smooth muscle elements at the onset of voiding, which can be observed on MRI, is likely to shorten and open up the prostatic urethra.

To investigate if in vitro contractile strength of the prostate and the prostatic urethra might correlate with the shortening of the ventral prostate seen on real-time magnetic resonance imaging (rtMRI). Micturition is a complex process that includes anatomical and neurological interactions for successful voiding. Recently we described on rtMRI that vertical contraction of the ventral prostate precedes initiation of male micturition and may contribute to the funnelling of the bladder neck.

In all, 10 patients undergoing radical prostatectomy (RP) were enrolled. Approval was obtained from all patients and by the local Ethics Committee. Preoperative rtMRI during voiding was performed as described before in eight patients undergoing RP, measuring the difference of the cranio-caudal distance of the ventral prostate (VP). To roughly estimate the amount of force required to deform the prostate in a vertical direction as seen on rtMRI, we uniaxially compressed the organ immediately after surgery by the same distance, assuming incompressibility and isotropy of prostatic tissue. A muscle strip (3 × 3 mm) from the ventral prostate, dorsal prostate and prostatic urethra was obtained after pathological evaluation. Contraction was elicited by electrical-field stimulation (EFS: 0.1 ms pulses at 2, 4, 8, 16, 32 and 64 Hz for 4 s).

There was a mean cranio-caudal contraction of the ventral prostate by 7.6 mm at the onset of micturition on rtMRI (P = 0.002). The mean (sd) contractile force of strips elicited by EFS at 32 Hz was 1472.44 (706.88) mN for the ventral prostate, 1044.24 (894.66) mN for the dorsal prostate, and 639.10 (785.06) mN for the prostatic urethra (P = 0.02). Extrapolating these values to the whole organ diameter, we calculated comparable force as observed in compression experiments.

Our functional and biometric in vitro analyses of prostate tissue showed sufficient contractile strength of the ventral prostate to induce a shortening of the organ as seen on rtMRI. There was significant higher contractile strength in the ventral prostate than in the dorsal prostate or the proximal urethra. The consistency of in vivo and in vitro results underlines the significance of the ventral prostate for the initiation of normal micturition.