Aim: 

To study the presence and underlying mechanisms of spontaneous Ca²⁺ oscillations in smooth muscle cells from urinary bladder of guinea pigs.

Methods: 

Detrusor cells of the urinary bladder were isolated from adult and newborn (10-13 days old) guinea pigs using collagenase and papain. After loading with fura-2 AM (4 mM, 20 min), cells were studied using a fluorescence digital microscope.

Results: 

Spontaneous Ca²⁺ oscillations were only rarely found in detrusor cells from adult individuals. In contrast, 35% of studied muscle cells from newborn guinea pigs displayed spontaneous [Ca²⁺]_i oscillations with several kinetic patterns (from irregular to highly paced cycles). The oscillations were sensitive to Ca²⁺ removal and to L-type and T-type Ca²⁺ channel blockers. Both the K⁺ channel opener pinacidil and the Ca²⁺ -activated Cl^- channel blocker NPPB also inhibited oscillations. Ca²⁺ stores were necessary for oscillations, as indicated by the inhibitory effects of thapsigargin, ryanodine and 2-APB. By the contrary, oscillations were enhanced by iberiotoxin, a specific blocker of Ca²⁺ -activated K⁺ channels. Oscillations were also inhibited by folimycin, an inhibitor of acidic NAADP-associated Ca²⁺ stores, but were enhanced by the cell-permable NAADP agonist RR-120.

Conclusion: 

Detrusor cells from newborn individuals develop spontaneous [Ca²⁺]_i oscillations due to Ca²⁺ influx through L-type Ca²⁺ channels, probably modulated by T-type Ca²⁺ channels and Ca²⁺ -activated Cl^- channels. This signal requires functional intracellular stores, including acidic pools associated to NAADP. This activity could explain the increased miogenic activity of detrusor cells of newborn individuals.

Supported by RETICEF, BFU 2007-60563 and PRI 07A069.