Age-related changes in Ca2+ channel antagonist receptors in rabbit lower urinary tract

Detrusor Underactivity: The Current Concept of the Pathophysiology

Based on evidence from available literature, we review the pathophysiology of detrusor underactivity (DU). DU is likely to be multifactorial. Aging reduces detrusor activity, but other concomitant causes may aggravate this condition, resulting in decrease of detrusor contractility. Impaired detrusor contractility has been regarded as a major etiologic factor of DU. However, a more complex pathology has been proposed. As contributing factors to DU, we discuss disturbances of the sensory afferent side of the micturition reflex, the central nervous system (CNS) and the efferent side of the reflex, including nerves and the detrusor muscle. Particularly, dysfunction of afferent nerves in the bladder and urethra may play a crucial role in the pathogenesis of DU. In addition, recent studies suggest that chronic bladder ischemia and resultant oxidative stress cause detrusor overactivity progressing to DU and inability to empty the bladder.

EFFECT OF AGE ON RABBIT BLADDER FUNCTION AND STRUCTURE FOLLOWING PARTIAL OUTLET OBSTRUCTION

PURPOSE

We determined whether young and old rabbits respond differently to partial bladder outlet obstruction.

MATERIALS AND METHODS

A total of 16 male New Zealand White rabbits were separated into 2 groups of 8 each. Group 1 consisted of young rabbits (age 7 weeks) and group 2 consisted of old rabbits (age 2 years). Four rabbits per group underwent partial outlet obstructions and 4 underwent sham operation. Four weeks following surgery individual bladder strips were used for contractile studies and the remaining tissue was examined histologically.

RESULTS

Contractile responses to all forms of stimulation between the young and old sham operated groups were similar. Contractile responses to all forms of stimulation were significantly decreased to the same degree in the 2 obstructed groups. However, the rate of tension generation to field stimulation was decreased to a significantly greater degree in young vs old bladders. Although young and old bladders showed smooth muscle hypertrophy, older rabbits showed significantly greater thickening of the serosa than young rabbits. Young rabbits showed significant inflammation, hemorrhage and expansion of the lamina propria, whereas old rabbits showed none of these characteristics.

CONCLUSIONS

Although there were only minor differences in the physiological response of young and old bladders to obstruction, young rabbits showed a significantly greater degree of histological damage. This may have been due to the thinner wall and greater sensitivity to distention.

L-type Ca2+ channels in fetal and adult ovine cerebral arteries

Recently, we reported that, whereas in cerebral arteries of the adult a majority of norepinephrine (NE)-induced increase in intracellular Ca2+ concentration ([Ca2+]i) comes from release of the sarcoplasmic reticulum (SR) Ca2+ stores, in the fetus the SR Ca2+ stores are relatively small, and NE-induced increase in [Ca2+]i results mainly from activation of plasma membrane L-type Ca2+ channels (20). In an effort to establish further the role of L-type Ca2+ channels in the developing cerebral arteries, we tested the hypothesis that, in the fetus, increased reliance on plasmalemmal L-type Ca2+ channels is mediated, in part, by increased L-type Ca2+ channel density. We used 3H-labeled (+)isopropyl-4-(2,1,3-benzoxadiazol-4-y1)-1,4-dihydro-(2,6-dimethyl-5-methoxycarbonyl)pyridine-3-carboxylate (PN200-110, isradipine) to measure L-type Ca2+ channel density (Bmax) in the cerebral arteries, common carotid artery (CCA), and descending aortae of fetal (approximately 140 gestation days), newborn (7-10 days), and adult sheep. In the cerebral and common carotid arteries, B(max) values (fmol/mg protein) of fetuses and newborns were significantly greater than those of adults. Western immunoblotting assay also revealed that the density of L-type Ca2+ channel protein in the cerebral arteries and CCA was about twofold greater in the fetus than the adult. Finally, compared with the adult, fetal cerebral arteries demonstrated a significantly greater maximum tension and [Ca2+]i in response to stimulation with the L-type Ca2+ channel agonist Bay K 8644. In addition, Bay K 8644-stimulated fetal vessels demonstrated a maximal tension and [Ca2+]i similar to that observed in response to stimulation with 10(-4) NE. These results support the idea that fetal cerebrovascular smooth muscle relies more on extracellular Ca2+ and L-type Ca2+ channels for contraction than does the adult and that this increased reliance is mediated, in part, by greater L-type Ca2+ channel density. This may have important implications in the regulation of cerebral blood flow in the developing organism.

Age-related changes in cholinergic and purinergic neurotransmission in human isolated bladder smooth muscles

We evaluated the correlation among age, cholinergic and purinergic neurotransmissions in the electrical field stimulation-induced contractions in human isolated urinary bladder smooth muscles, using the muscle bath technique. Human bladder specimens were divided into three groups (G1, under 50years; G2, 51-70years; G3, over 70years old), and each muscle strip was suspended in a thermostatically controlled organ bath filled with oxygenated Krebs-Henseleit solution, connected to an isometric force displacement transducer, and an isometric tension development was recorded. The contractile responses induced by KCl, carbachol, adenosine triphosphate (ATP) and electrical field stimulation, and the effects of atropine and alpha, beta methylene ATP on electrical field stimulation-induced contractions were observed. The contractile response to KCl and concentration-response curves for carbachol and ATP, and frequency-response curves for electrical field stimulation were not significantly different among the three groups. The atropine sensitive and resistant parts of contraction induced by electrical field stimulation were decreased and increased with age, respectively. There are significant positive and negative correlations between age and the purinergic, and age and the cholinergic neurotransmissions in human isolated bladder smooth muscles, respectively. The age-related changes in neurotransmissions may contribute to the changes in bladder function in the elderly.

Effect of age and outlet resistance on rabbit urinary bladder emptying

PURPOSE

To investigate the influence of age and effect of increased outlet resistance on the ability of rabbit bladders to empty in response to various methods of stimulation.

MATERIALS AND METHODS

Bladders from six-month-old (young) and three-year-old rabbits (aged) were mounted in an in vitro whole organ bath system and filled with 15 ml. saline. The ability of the bladders to empty against low outlet resistance (LOR) and high outlet resistance (HOR) in response to field stimulation, bethanechol, and KCl was measured. The following parameters were measured: intravesical pressure and volume emptied. From these, flow rate, power, and external mechanical work were calculated.

RESULTS

Maximum isometric pressure did not change with age. All bladders emptied with increased pressure and decreased flow rate at HOR. The young bladders generated a greater maximum power in response to bethanechol and KCl than the aged bladders at both outlet resistances, and maximum power did not change with increased resistance. The aged bladders did less work and emptied significantly less than the young bladders at the HOR.

CONCLUSIONS

The aged rabbit bladders were unable to maintain the bladder contraction long enough to empty completely through an increased outlet resistance. Because maximum power remained constant when the outlet resistance was increased, it might be useful clinically to determine the emptying ability of the urinary bladder, independent of changes in outlet resistance. In addition, bladder work could be used to evaluate bladder function if the volume emptied is also taken into consideration.

Old age does not affect shortening velocity or content of contractile and cytoskeletal proteins in the rat detrusor smooth muscle

The influence of old age on mechanical properties of the urinary bladder was investigated using smooth muscle strips from urinary bladders of control (14-16 weeks) and old-age (104 weeks) female Sprague-Dawley rats. Bladder weight of the aged rats had increased by about 30%. The maximal shortening velocity and stiffness in skinned activated urinary bladder fibers from old animals were unchanged compared to controls. The relative content of intermediate filament proteins to actin and the relative content of myosin to actin was unchanged. The concentration of myosin was unchanged (about 6.5 microg/mg wet weight). The results suggest that old age is not associated with pronounced changes in the cellular contractile and cytoskeletal proteins or in the mechanical properties of the contractile machinery. The age-related changes in mechanical properties previously reported for intact smooth muscle from urinary bladder are most likely due to alterations in the activation systems.

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.