Nicardipine as a Ca2+ channel blocker in single barnacle muscle fibers

Extracellular Ca2+ increases cytosolic free Ca2+ in freshly isolated rat odontoblasts

Recent evidence suggests that extracellular Ca2+ may modulate cell function in mineralized tissue. To determine whether dentinogenic cells, in particular, are sensitive to extracellular Ca2+, fura-2 microfluorometry was used to monitor intracellular calcium levels in odontoblasts freshly isolated from rat incisor. In response to applications of 0.5-4.0 mM extracellular calcium (CaCl2), most odontoblasts (84%; 107/128) showed an increase in intracellular calcium. For the majority of these cells (70%; 75/107), the typical response was biphasic; there was an initial, transient increase in intracellular calcium which reached peak levels within 30-50 s and decayed rapidly, followed by a slower (> 300 s) recovery toward basal levels. In general, the response of these cells to calcium was repeatable and the mean calcium concentration for the half-maximal response was approximately 1.3 mM. This effect could be partially blocked by either 200 microM lanthanum, a nonspecific blocker of Ca2+ channels, or 20 microM dantrolene, a potent inhibitor of Ca2+ release from internal stores. Used in combination, lanthanum, and dantrolene nearly abolished the calcium response completely. In addition, this response was sensitive to the dihydropyridine-sensitive calcium channel blocking agent nicardipine (60 microM), indicating a role for voltage-gated calcium channels during these events. These results show that odontoblasts respond to external calcium through mechanisms involving both influx of external calcium as well as release of calcium from internal stores and suggest a role for extracellular calcium in regulating the function of these cells.

Effect of pentachlorophenol on calcium accumulation in barnacle muscle cells

1. The effect of extracellularly applied pentachlorophenol (PCP) was studied on the membrane potential (Vm) and Ca2+ uptake in isolated single skeletal muscle cells of Balanus nubilus. 2. When compared with the controls, 0.1 mM PCP induced a significant (P < 0.05) increase in Ca2+ uptake accompanied by membrane depolarization (9 mV at 45 min incubation). This depolarization was reduced by 11% of extracellular Ca2+ (Cao2+) was replaced by Tris+ and by 50% if extracellular Na+ was also replaced by Tris+. 3. The Ca2+ channel blocker, verapamil (0.1 mM), completely inhibited the PCP-induced Ca2+ uptake as well as the membrane depolarization either in the absence or presence of Cao2+. Experiments on voltage-clamped cells show that the PCP-induced Ca2+ uptake was independent of the PCP-induced depolarization. 4. The results indicate that PCP induces activation of a verapamil-sensitive Ca2+ influx pathway (presumably L-type Ca2+ channels) independent of Vm. The permeation of Ca2+, Na+ and Tris+ through this pathway produces membrane depolarization in the following order of effectiveness: Ca2+ > Na+ > Tris+.

Opposite roles of cAMP and cGMP on volume loss in muscle cells

It is controversial whether changes in adenosine 3',5'-cyclic monophosphate (cAMP) and in the cAMP-to-guanosine 3',5'-cyclic monophosphate (cGMP) ratio are involved with cell swelling and in the activation of volume-regulatory mechanisms. We examined whether these nucleotides are involved in cell volume regulation in skeletal muscle. Isolated (intact and internally perfused) barnacle muscle cells were used because these cells, when exposed to a hyposmotic environment, undergo an extracellular Ca2+ (Cao)-dependent regulatory volume decrease (RVD). Using intact cells we found that dibutyryl cAMP and forskolin significantly promoted RVD in cells exposed to Cao-free solutions and that dibutyryl cGMP significantly inhibited RVD in cells exposed to Cao-containing solutions. In perfused cells in which the intracellular free Ca2+ concentration ([Ca2+]i) was heavily buffered [with 8 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA)], cAMP induced a volume loss that was inhibited by presence of cGMP. Furthermore, if perfused cells were exposed to hyposmotic conditions, they swelled and underwent RVD provided that [Ca2+]i buffering was low (with 2 mM EGTA). This effect was inhibited by presence of the cAMP antagonist, [R]-p-adenosine 3',5'-cyclic monophosphorothioate.

Changes in membrane potential associated with cell swelling and regulatory volume decrease in barnacle muscle cells

Our aim was to test the effect of hypotonicity and extracellular Ca2+ (Cao) on cell volume and membrane potential (VM) in barnacle muscle cells. Under isotonic conditions the resting VM of isolated cells mounted in the experimental chamber exposed to either Ca(2+)-free or Ca(2+)-containing (11 mM) solutions was -46.3 +/- 1.0 mV (n = 24) and -56.2 +/- 0.9 mV (n = 38), respectively. In the absence of Cao, the cells depolarized at a rate of 2.3 +/- 0.47 mV/hr; the presence of Cao reduced this rate of depolarization by 2.9-fold. Both in the absence or presence of Cao, the cells swelled in response to hypotonicity but underwent regulatory volume decrease (RVD) when Cao was present. Addition of the Ca2+ channel blocker, verapamil (0.1 mM), inhibited the Cao-dependent RVD. The percentage of cells responding with RVD increased with larger hypotonic challenges. There was a Cao-independent direct relationship between cell swelling and membrane depolarization which can be explained by dilution of the concentration of intracellular K+ ([K+]i). RVD was accompanied by a small hyperpolarization (3.0 +/- 0.38 mV/2 hr) which may represent increases in [K+]i during cell shrinking and activation of a conductive pathway. The results indicate the following: (1) the presence of Cao stabilizes VM; (2) cell swelling produces a depolarization which can be explained by dilution of [K+]i; (3) cell swelling activates a verapamil-sensitive Ca2+ influx responsible for promoting RVD; and (4) RVD is accompanied by a hyperpolarization which may result from activation of a conductive pathway.

Abolition with chloramine-T of inactivation in barnacle muscle fibers results in stimulation of the ouabain-insensitive sodium efflux

The hypothesis that chloramine-T stimulates the basal Na+ efflux in barnacle fibers as the result of the entry of trigger Ca2+ into the myoplasm from the bathing medium was examined in this study. Two reasons for doing so can be given. One is that the oxidant is known to abolish inactivation in sodium and potassium channels. The other is that L-type Ca2+ channels are present in barnacle fibers, and an increase in internal free Ca2+ in these fibers is known to stimulate the Na+ efflux, particularly in ouabain-poisoned fibers. The results of the experiments are as follows: (i) Chloramine-T exerts a biphasic effect on the Na+ efflux: inhibition is followed by stimulation, the threshold concentration being 10(-5) M. This is also found to be the threshold concentration for shortening of these fibers. (ii) The kinetics of the inhibitory effect resemble those of ouabain. (iii) Ouabain is without effect on the stimulatory phase caused by chloramine-T. (iv) Application of chloramine-T after the full effect of 10(-4) M-ouabain is reached elicits solely a stimulatory response. (v) The dose-response curves for the stimulatory action of chloramine-T in unpoisoned and ouabain-poisoned fibers are alike except that the threshold concentration is less than 10(-5) M in poisoned fibers. (vi) Basal light emission from unpoisoned and ouabain-poisoned fibers loaded with the photoprotein, aequorin, some 60 min beforehand increases as soon as they are exposed to 10(-4) M chloramine-T. The response recorded in unpoisoned fibers is monophasic and usually transitory, whereas it is multiphasic and usually sustained in ouabain-poisoned fibers. (vii) The dose-response curve for chloramine-T shows a shift to the left in poisoned fibers. (viii) The magnitude of the rise in light emission depends on the external Ca2+ concentration. A rise fails to take place in the nominal absence of external Ca2+. Taken together, these results support the above hypothesis that chloramine-T causes the entry of trigger Ca2+ into the myoplasm from the outside and provide evidence that stimulation of the Na+ efflux is associated not only with this event but also with a reduced Na+ gradient resulting from inhibition of the membrane Na+/K(+)-ATPase system by the oxidant. Thus, the suggestion put forward is that this oxidant promotes reverse Na+/Ca2+ exchange and is able to exert multiple effects on membrane transport.

Lack of effect on the sodium efflux of the microinjection of D-Ins(1,4,5)P3 into ouabain-poisoned barnacle muscle-fibers

A study has been carried out using relatively intact mature muscle fibers from the barnacle Balanus nubilus to see whether D-Ins(1,4,5)P3 stimulates the ouabain-insensitive Na efflux following its microinjection and whether this is accompanied by a contraction of the fiber. Part of the impetus for a study of this type came from the on-going debate between Vergara, Rojas and co-workers and Lea and co-workers, the former group holding the view that skinned barnacle fibers and skeletal fibers in general are responsive to this isomer. The evidence brought forward indicates that the injection of D-Ins(1,4,5)P3 in concentrations in the range of 10(-2) M to 10(-6) M into cannulated unskinned fibers pretreated with ouabain fails to increase the residual efflux, and additionally fails to elicit a contraction. A similar picture emerges with the use of non-hydrolyzable DL-Ins(1,4,5)P3[S]3, analog following its injection in a concentration of 0.5 microM. Higher concentrations of the analog were unavailable for use. This work also involved verification of the idea that an effect might be obtainable in depolarized fibers. However, doubling or tripling K0+ and injection of the isomer or the analog simultaneously failed to support this idea. Since nothing is known as to whether D-Ins(1,4,5)P3 influences the behavior of the Na(+)-Ca2+ exchanger when operating in the reverse mode, experiments were done to check this possibility. ATPNa2 which is though to activate Na(+)-Ca2+ exchange was injected prior to the isomer or the analog but no significant results were obtained. A similar line of reasoning was followed, that of activating the L-type Ca2+ channel by injecting GTPNa2 which in addition is known to activate adenylate cyclase. Again, neither the isomer nor the analog were effective. Thus, the only conclusion possible is that in relatively intact, mature barnacle fibers there is no coupling between the phosphoinositide signalling pathway and two other key systems, viz. the Na(+)-Ca2+ exchanger when operating in the reverse mode and the L-type Ca2+ channel. Equally clear is that for some unknown reason the ouabain-insensitive Na efflux and the contractile apparatus are insensitive to D-Ins(1,4,5)P3[S]3.

An investigation of the sensitivity of the ouabain-insensitive sodium efflux in single barnacle muscle fibers to pentachlorophenol

The aim of the present work was to explore the possibility that pentachlorophenol (PCP) influences the behavior of the resting Na efflux in single muscle fibers from the barnacle, Balanus nubilus. It is shown here that PCP causes a transitory rise in the Na efflux in both unpoisoned and ouabain-poisoned fibers and that the response is dose-dependent, the minimal effective concentration in ouabain treated fibers being less than 10(-6) M. The efficacy of PCP is significantly greater than that of 2,3,4-trichlorophenol. 2,3-Dichlorophenol is ineffective. This is also the case with phenol. The magnitude of the response to PCP is a function of external pH. Lowering pHe increases the response. The response has an absolute requirement for external Ca2+ and is a sigmoidal function of external Ca2+ concentration. Since treatment of these fibers with PCP in high concentration leads to prompt contraction, experiments were designed to determine whether the observed rise in ouabain-insensitive Na efflux is due to a fall in myoplasmic pCa and whether trigger Ca2+ originates from the bathing medium. The results obtained show that prior injection of ethylene glycol bis(beta-aminoethyl ether) N,N'-tetraacetic acid (EGTA) or 1,2-bis(2-aminophenoxyethane-N,N,N',N'-tetraacetic acid (BAPTA) leads to a drastic reduction in the response to PCP. They also show that prior external application of verapamil or devapamil stops the response to PCP from occurring. Both Cd2+ and Co2+ are also effective but only temporarily. Last, the effects of ryanodine and 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) were tested, since the former is known to block the sarcoplasmic reticulum Ca2+ release channel, and the latter to impair the action of agents known to release Ca2+ from internal depots. Both ryanodine and TMB-8 are found to reduce the response to PCP. Taken together, these observations support the hypothesis that PCP stimulates the ouabain-insensitive Na efflux by increasing the internal free Ca2+ and that the increase in internal Ca2+ is due to the entry of trigger Ca2+ from the outside via Ca2+ channels, as well as release of Ca2+ by the sarcoplasmic reticulum via its channel. They also indicate that the efficacy of PCP depends on the 5 Cl atoms present in its aromatic ring and pHe.