Perspective on the role of P2X-purinoceptor activation in human vas deferens contractility.

The contractile actions of α,ß-methylene ATP (α,ß-meATP) and ATP and the effects of K(+) channel blockers in longitudinal and circular muscles of human vas deferens were investigated with a view to clarifying the functional importance of P2X(1)-purinoceptor activation and K(+) channels in modulating contractility of the tissues. The results provide an experiment-based perspective for resolving differing reports on purinergic activation of the tissues and uncertain roles of large-conductance Ca(2+)-activated K(+) (BK(Ca)) and voltage-gated delayed rectifier K(+) (K(V)) channels. α,ß-Methylene ATP (3-100 µm) evoked suramin-sensitive contractions of longitudinal muscle but rarely of circular muscle. ATP (0.1-3 mm) less reliably activated only longitudinal muscle contractions. These were enhanced by ARL 67156 (100 µm), but a different ectonucleotidase inhibitor, POM 1, was ineffective. Both muscle types were unresponsive to ADP-ßS (100 µm), a P2Y-purinoceptor agonist. Longitudinal muscle contractions in response to α,ß-meATP were enhanced by FPL 64176 (1 µm), an L-type Ca(2+) agonist, TEA (1 mm), a non-specific K(+) channel blocker, 4-aminopyridine (0.3 mm), a selective blocker of K(V) channels, and iberiotoxin (0.1 µm), a selective blocker of BK(Ca) channels. Quiescent circular muscles responded to α,ß-meATP reliably in the presence of FPL 64176 or iberiotoxin. Apamin (0.1 µm), a selective blocker of small conductance Ca(2+)-activated K(+) (SK(Ca)) channels had no effect in both muscle types. Y-27632 (1-10 µm) reduced longitudinal muscle contractions in response to α,ß-meATP, suggesting involvement of Rho-kinase-dependent contractile mechanisms. The results indicate that P2X(1)-purinoceptor stimulation elicits excitatory effects that: (a) lead to longitudinal muscle contraction and secondary activation of 4-aminopyridine-sensitive (K(V)) and iberiotoxin-sensitive (BK(Ca)) K(+) channels; and (b) are subcontractile in circular muscle due to ancillary activation of BK(Ca) channels. The novel finding of differential action by P2X(1)-purinoceptor agonists in the muscle types has functional implication in terms of the purinergic contribution to overall contractile function of human vas deferens. The modulatory effects of K(V) and BK(Ca) channels following P2X(1)-purinoceptor activation may be pivotal in providing the crucial physiological mechanism that ensures temporal co-ordination of longitudinal and circular muscle contractility.

Contractile actions of L-type Ca2+ agonists in human vas deferens and effects of structurally different Ca2+ antagonists.

The actions of L-type Ca(2+) agonists, FPL 64176 and Bay K 8644 were investigated in human vas deferens in the presence of structurally different L-type Ca(2+) antagonists. The L-type Ca(2+) agonists (or=20 min) evoked only rhythmic contractility even in moderately depolarizing ([K(+)](o), 10mM) medium. These findings suggest low basal activity of L-type Ca(2+) channels (VOCs) in both muscle types. In the presence of L-type Ca(2+) agonists (1 microM), high [K(+)](o) (30 or 120 mM) evoked contractions with different profiles. Circular muscle had a predominance of rhythmic activity ([K(+)](o) 30 mM) and slow time to peak and decline ([K(+)](o) 120 mM). Longitudinal muscle was more tonic ([K(+)](o) 30 mM) with a rapid time to peak and decline ([K(+)](o) 120 mM). The contractions in both muscle types were blocked by nifedipine or methoxyverapamil; indicating the involvement of L-type VOCs and suggests that the distinct contractile profiles originate from differences in mechanisms that regulate contractility. In comparison to the conventional L-type Ca(2+) antagonists, fendiline, prenylamine and thioridazine were more effective against longitudinal than circular muscle contractions. Structurally similar diphenylalkylamines (cinnarizine, flunarizine, and pimozide) and phenothiazines (sulphoridazine, chlorpromazine, and trifluoperazine) inhibited the contractions comparably in both muscle types. These findings are discussed in relation to inhibition of muscle type-specific mechanisms that may contribute more to L-type VOC activation and contractility in longitudinal than in circular muscle.

Strength, power output and symmetry of leg muscles: effect of age and history of falling.

Risk factors for medically unexplained falls may include reduced muscle power, strength and asymmetry in the lower limbs. Conflicting reports exist about strength and there is little information about power and symmetry. Forty-four healthy young people (29.3 +/- 0.6 years), 44 older non-fallers (75.9 +/- 0.6 years), and 34 older fallers (76.4 +/- 0.8 years) were studied. Isometric, concentric and eccentric strength of the knee and ankle muscles and leg extension power were measured bilaterally. The younger group was stronger in all muscles and types of contraction than both older groups (P < 0.02-0.0001). Strength differences between the older groups occasionally reached significance in individual muscles and types of contraction but overall the fallers had 85% of the strength and 79% of the power of the non-fallers (P < 0.001). Young subjects generated more power than both older groups (P < 0.0001) and the fallers generated less than the non-fallers (P = 0.03). Strength symmetry showed an inconsistent age effect in some muscles and some contraction types. This was similar overall in the two older groups. Both older groups had greater asymmetry in power than the young (P < 0.02-0.004). Power asymmetry tended to be greater in the fallers than the non-fallers but this did not reach significance. These data do not support the suggestion that asymmetry of strength and power are associated with either increasing age or fall history. Power output showed clear differences between age groups and fall status and appears to be the most relevant measurement of fall risk and highlights the cumulative effects on function of small changes in strength in individual muscle groups.

Steadiness of quadriceps contractions in young and older adults with and without a history of falling.

Decreased steadiness of muscle force may be associated with ageing and could be a cause of falls in older people. We studied this in isometric and anisometric quadriceps contractions in healthy young and older people. The older group contained people with and without a history of medically unexplained falls. Forty-four young (aged 18-40 years) and 78 older (aged > 70 years) subjects participated. In the latter group 34 people had a history of falling (fallers) and 44 did not (non-fallers). Isometric steadiness was measured by the coefficient of variation (CoV) of force at 10, 25 and 50% maximal voluntary force (MVC). Anisometric steadiness was measured by the SD of acceleration during concentric and eccentric contractions against two external loads (1 and 5 kg). There was an overall trend for the younger subjects to be most steady and the fallers the least but the differences were not consistently significant. Isometric steadiness was unaffected by force in all groups. The fallers were less steady (P < 0.001) than both the young and non-fallers, who had similar values. During anisometric contractions, steadiness was similar with both external loads and types of contraction in all groups. During dynamic contractions the older subjects were less steady (P < 0.002). Only eccentric contractions distinguished between the two older groups, with the fallers being less steady by 31% (P = 0.013). These data indicate ageing per se is associated with decreased anisometric, but not isometric, steadiness. Greater unsteadiness during eccentric contractions in the fallers could be an important mechanism of medically unexplained falls.

Fatigue and functional performance of human biceps muscle following concentric or eccentric contractions.

A long-lasting fatigue was measured in human biceps muscle, following 40 maximal isokinetic concentric or eccentric contractions of the forearm, as the response to single-shock stimuli every minute for 4 h. This protocol allowed new observations on the early time course of long-lasting fatigue. Concentric contractions induced a novel progressive decline to 30.2% (SE 7.8, n = 7) of control at 23 min with complete recovery by 120 min. Eccentric contractions lead initially to a smaller force reduction of similar time course followed by a slower decline to 40.0% (SE 5.1, n = 7) control at 120 min with recovery less than half complete at 4 h. A 50-Hz test stimuli overcame both fatigues, identifying low-frequency fatigue. EMG recordings from the biceps muscle showed moderate (<20%) changes during the fatigue. A visual-tracking task showed no decrement in performance at the time of maximal fatigue of the single-shock response. Because the eccentric contractions have a similar activation, a larger force, but much smaller metabolic usage than concentric contractions, it is concluded that the initial decline is related to the effects of metabolites, whereas the slower phase after eccentric contractions is associated with higher mechanical stress.

Contractile actions of imidazoline alpha-adrenoceptor agonists and effects of noncompetitive alpha1-adrenoceptor antagonists in human vas deferens.

The contractile actions of imidazoline alpha-adrenoceptor agonists were investigated in human vas deferens longitudinal and circular muscle. The effects of phenoxybenzamine were studied in comparison to dibenamine and SZL-49 (4-amino-6,7-dimethoxy-2-quinazolinyl-4-(2-bicyclo[2,2,2]octa-2,5-dienylcarbonyl-2-piperazine), an alkylating prazosin analogue that discriminates between alpha(1H)- and alpha(1L)-adrenoceptor subtypes. The imidazoline alpha-adrenoceptor agonist, A-61603 (N-[5-(4,5-dihydro-1H-imidazol-2yl)-2-hydroxy-5,6,7,8-tetrahydronaphthalen-1-yl]methanesulfonamide hydrobromide), was a potent agonist (pD(2); longitudinal muscle 6.9, circular muscle 6.4) and cirazoline a partial agonist (pD(2); longitudinal muscle 6.1, circular muscle 5.1). Oxymetazoline was less effective, indanidine and clonidine were ineffective. SZL-49 produced a differential inhibition of contractions evoked by A-61603 in circular (alpha(1H)) compared to longitudinal (alpha(1L)) muscle and phenoxybenzamine had the opposite effect. Dibenamine inhibited the contractions comparably in both muscle types and analyses of its partial alkylation of receptors yielded identical estimates of equilibrium dissociation constant (pK(d)) for A-61603 in longitudinal (5.82) and circular (5.84) muscle. Receptor occupancy-response relationships revealed that whilst the muscle types are not different in receptor reserves for A-61603, contraction to the potent imidazoline is more efficiently coupled in longitudinal than in circular muscle. This underlies the markedly different responsiveness of the muscle types to cirazoline or oxymetazoline (alpha-adrenoceptor agonists with lower efficacies relative to A-61603). The differential inhibitory actions of phenoxybenzamine and SZL-49 are discussed.

Discrimination by SZL49 between contractions evoked by noradrenaline in longitudinal and circular muscle of human vas deferens.

The effects of irreversible alpha1-adrenoceptor antagonists, SZL-49 (an alkylating analogue of prazosin), dibenamine and benextramine on contractions to noradrenaline (NA) in longitudinal and circular muscle of human epididymal vas deferens were investigated. Competitive alpha1-adrenoceptor antagonists were also used to further characterize the alpha1-adrenoceptor subtype stimulated by NA in longitudinal and circular muscle. NA evoked concentration-dependent contractions of both muscle types (pD2; 5.4 and 5.2 respectively). The contraction of circular muscle was comparatively more sensitive than that of longitudinal muscle to pretreatment with SZL-49. In contrast, dibenamine or benextramine produced comparable effects in both muscle types. The relationship between receptor occupancy and contraction in either longitudinal or circular muscle was nonlinear, with half-maximal response requiring similar receptor occupancy (longitudinal muscle 14%, circular muscle 16%). Maximal response in both muscle types occurred with little or no receptor reserve (<10%). The competitive alpha1-adrenoceptor antagonists produced dextral shifts of the dose-response curves to NA in longitudinal and circular muscle. The inhibitory potencies, estimated from the apparent pKB values were significantly different in longitudinal and circular muscle respectively for either WB 4101 (pKB, 8.6 and 9.5) or RS-17053 (pKB, 7.1 and 9.0) but not for Rec 15/2739 (pKB, 9.2 and 9.8) or HV 723 (pKB, 8.3 and 8.4). In conclusion, the potency profile of the competitive alpha1-adrenoceptor antagonists and the lack of different receptor reserves for NA in the muscle types suggest that the discriminatory effects of SZL-49 is primarily due to a predominance of the alpha1L-adrenoceptor subtype in longitudinal muscle and alpha1A-subtype in circular muscle.