Characterization of the spontaneous electrical and contractile activity of smooth muscle cells in the rat upper urinary tract

Purpose: We morphologically and electrophysiologically identified the cells that generate the electrical activity underlying the peristaltic contractions of the rat upper urinary tract. Materials and Methods: Electron microscopy and tension recording techniques were used to characterize the smooth muscle cells underlying spontaneous contractions in the wall of the rat ureter, and proximal and distal renal pelvis. Intracellular microelectrodes, containing 4% neurobiotin were used to record data from the cells of the renal pelvis, which were later viewed on a confocal microscope. Results: Spontaneous myogenic contractions (average 22.3 +/- 2.2 minutes(-1)) originated in the proximal renal pelvis and propagated into the distal renal pelvis and ureter in 6 preparations. Smooth muscle cells in the renal pelvis and ureter were typical in appearance with greater than 85% of their sectional area containing clumped contractile filaments. In contrast, contractile fibrils occupied only 65% of the sectional area of the smooth muscle cells within the most proximal region of the renal pelvis (pelvicaliceal junction). In strips of the renal pelvis spindle shaped cells 83 to 200 mum. long fired spontaneous action potentials (6 minutes(-1)) consisting of an initial spike, a quiescent plateau phase and abrupt hyperpolarization to a peak diastolic potential of -60 mV. Other spindle shaped cells 94 to 112 mum. long displayed small membrane transients (15 minutes(-1)) 9 to 19 mV. in amplitude, firing from a diastolic potential of -40 mV. Conclusions: It is likely that the spontaneous contractile activity of the rat upper urinary tract arises from the discharge of action potentials in typical smooth muscle cells of the proximal renal pelvis that are directly driven by the spontaneous membrane oscillations of atypical smooth muscle cells.