Age-related differential susceptibility to calcium channel blocker and low calcium medium in rat detrusor muscle: response to field stimulation.

The influences of aging on the response of urinary bladder detrusor smooth muscle in response to neurogenic stimulation (electric field stimulation) were investigated in rats aged 3-4 months (young) and 18-20 months (old). In addition, age-associated changes in calcium homeostasis were evaluated by comparing the basal intracellular free calcium concentration ([Ca2+]i), the change of [Ca2+]i in response to field stimulation, the contractile response at different extracellular calcium concentrations (Ca2+ = 1.8, 0.6, and 0 mM), and the sensitivity to the L-type calcium channel blocker. In both groups, the frequency-response curve in response to field stimulation (1-32 Hz) was similar. The contractile response to field stimulation was biphasic (phasic and tonic). The phasic/tonic ratio at different frequencies was similar between the two groups. The contractile responses were equally inhibited by atropine (1 microM) and preincubation with alpha, beta-methylene ATP (50 microM). Tetrodotoxin (1 microM) virtually abolished the responses to field stimulation in both groups. The biphasic components showed different sensitivities to diltiazem and low calcium. The tonic component was more readily inhibited by diltiazem, whereas the phasic component was more sensitive to low calcium medium. In general, the response to lower frequency stimulations was more sensitive to both diltiazem and low calcium. The aged group was more sensitive to the inhibitory effects of both low calcium and diltiazem. In Ca2(+)-free medium, the contractile response to higher frequency (e.g., 32 Hz) was temporarily preserved in most of the preparations. This response disappeared after repeated stimulation. The magnitude of this Ca2(+)-free response to 32 Hz stimulation was significantly lower in the aged group than in the young group (18.1 +/- 1.4 vs.29.1 +/- 3.2% of the control response in normal Tyrode medium, P < 0.05). The basal [Ca2+]i levels were similar in the two groups (young: 131 +/- 12 nM, n = 5; old: 118 +/- 10 nM, n = 4, P > 0.05). The magnitude and time course of the [Ca2+]i response to field stimulation were also similar in the two groups. It is concluded that aging seems to have little effect on the neuromuscular activities of rat urinary bladder. However, the calcium homeostasis may be altered as evidenced by the increased sensitivity to diltiazem and low extracellular calcium.