How do we know when to empty our bladders? This mechanism has been explained by researchers at the University of Vermont College of Medicine. This discovery may also lead to new therapeutic interventions for bladder dysfunction.Sensing bladder fullness is seemingly simple. The kidneys send waste and excess water to the bladder, and upon reaching its filling threshold, the bladder tells the central nervous system that it's time to void. However, a team led by Mark T. Nelson, PhD, University Distinguished Professor and Chair of the Department of Pharmacology, found that in addition to filling pressure, the process involves what they call "non-voiding transient contractions (TCs)" of the urinary bladder smooth muscle.
Although filling pressure did not affect the frequency of TCs, it did increase the rate at which they reached their maximum pressure (rate of rise). This latter property reflects a change in the length-tension relationship of detrusor smooth muscle, an important biophysical property that determines how efficiently the muscle will contract. "This meant that the rate of rise of the TC tells the brain not only how full the bladder is, but also if the bladder muscle can contract sufficiently for normal voiding," Dr. Nelson said.
In addition to this, they found that inhibiting either small- or large-conductance calcium-activated potassium (SK and BK) channels-both of which are important in helping smooth muscle relax-increased TC amplitude and sensory nerve activity.
"We have known for years that BK channels in urinary bladder smooth muscle cells help determine excitability," Dr. Nelson said. "The more the channels are on, the less excitable the bladder smooth muscle becomes, the fewer of these transient contractions you have. But if we block SK channels, we get a much bigger burst of sensory nerve outflow. It looks like SK channels are in an interstitial cell type that is involved in sensing this small, but rapid, change in pressure."
The study, "Transient contractions of urinary bladder smooth muscle are drivers of afferent nerve activity during filling," by Thomas J. Heppner et al., appears in the April issue of The Journal of General Physiology.