Cerebral vascular smooth muscle potassium channels and their possible role in the management of vasospasm

Effects of Resistance Exercise and Nutritional Supplementation on Dynamic Cerebral Autoregulation in Head-Down Bed Rest

Head-down bed rest (HDBR) is commonly considered as ground-based analog to spaceflight and simulates the headward fluid shift and cardiovascular deconditioning associated with spaceflight. We investigated in healthy volunteers whether HDBR, with or without countermeasures, affect cerebral autoregulation (CA). Twelve men (at selection: 34 ± 7 years; 176 ± 7 cm; 70 ± 7 kg) underwent three interventions of a 21-day HDBR: a control condition without countermeasure (CON), a condition with resistance vibration exercise (RVE) comprising of squats, single leg heel, and bilateral heel raises and a condition using also RVE associated with nutritional supplementation (NeX). Cerebral blood flow velocity was assessed using transcranial Doppler ultrasonography. CA was evaluated by transfer function analysis and by the autoregulatory index (Mxa) in order to determine the relationship between mean cerebral blood flow velocity and mean arterial blood pressure. In RVE condition, coherence was increased after HDBR. In CON condition, Mxa index was significantly reduced after HDBR. In contrast, in RVE and NeX conditions, Mxa were increased after HBDR. Our results indicate that HDBR without countermeasures may improve dynamic CA, but this adaptation may be dampened with RVE. Furthermore, nutritional supplementation did not enhance or worsen the negative effects of RVE. These findings should be carefully considered and could not be applied in spaceflight. Indeed, the subjects spent their time in supine position during bed rest, unlike the astronauts who perform normal daily activities.

Serum Metabolomics Study Based on LC-MS and Antihypertensive Effect of Uncaria on Spontaneously Hypertensive Rats

Our previous studies have shown that Uncaria has an important role in lowering blood pressure, but its intervention mechanism has not been clarified completely in the metabolic level. Therefore, in this study, a combination method of HPLC-TOF/MS-based metabolomics and multivariate statistical analyses was employed to explore the mechanism and evaluate the antihypertensive effect of Uncaria. Serum samples were analyzed and identified by HPLC-TOF/MS, while the acquired data was further processed by partial least squares discriminant analysis (PLS-DA) and orthogonal partial least squares discriminant analysis (OPLS-DA) to discover the perturbed metabolites. A clear cluster among the different groups was obtained, and 7 significantly changed potential biomarkers were screened out. These biomarkers were mainly associated with lipid metabolism (dihydroceramide, ceramide, PC, LysoPC, and TXA2) and vitamin and amino acids metabolism (nicotinamide riboside, 5-HTP). The result indicated that Uncaria could decrease the blood pressure effectively, partially by regulating the above biomarkers and metabolic pathways. Analyzing and verifying the specific biomarkers, further understanding of the therapeutic mechanism and antihypertensive effect of Uncaria was acquired. Metabolomics provided a new insight into estimate of the therapeutic effect and dissection of the potential mechanisms of traditional Chinese medicine (TCM) in treating hypertension.

Multiple Actions of Phencyclidine and (+)MK-801 on Isolated Bovine Cerebral Arteries

This study examines the direct effects of 3 noncompetitive N-methyl-D-aspartate receptor antagonists, phencyclidine (PCP), (+)MK-801, and (-)MK-801, on bovine middle cerebral arteries (BMCA). Rings of BMCA were mounted in isolated tissue chambers equipped with isometric tension transducers to obtain pharmacologic dose-response curves. In the absence of endogenous vasoconstrictors, the 3 N-methyl-D-aspartate antagonists each produced direct constriction of BMCA. The thromboxane A2 receptor antagonist SQ-29,548, the TxA2 synthase inhibitor furegrelate, the calcium antagonist nimodipine, and calcium-deficient media all inhibited maximal phencyclidine or (+)MK-801-induced constriction. Direct constriction by PCP or (+)MK-801 was independent of the presence of endothelium. When BMCA were preconstricted with potassium-depolarizing solution, PCP, (+)MK-801, and (-)MK-801 each produced only concentration-dependent relaxation. When BMCA were preconstricted with the stable TxA2 analog U-46,619 and exposed to increasing concentrations of PCP, (+)MK-801, or (-)MK-801, tension increased. Thromboxane A2 may contract BMCA by acting as a potassium channel blocker; iberiotoxin and tetraethylammonium both constrict BMCA. In Ca-deficient media containing either potassium or U-46,619, phencyclidine and (+)MK-801 each produced competitive inhibition of subsequent Ca-induced constriction. In additional experiments, arterial strips were mounted in isolated tissue chambers to directly measure calcium uptake, using Calcium as a radioactive tracer. Both phencyclidine and (+)MK-801 blocked potassium-stimulated or U-46,619-stimulated Ca uptake into arterial strips. These results suggest that phencyclidine and (+)MK-801 have 2 separate actions on BMCA. They may constrict arterial rings by releasing TxA2 from cerebrovascular smooth muscle, and relax arterial rings by acting as calcium antagonists.

ACh-induced depolarization in inner ear artery is generated by activation of a TRP-like non-selective cation conductance and inactivation of a potassium conductance

Adequate cochlear blood supply by the spiral modiolar artery (SMA) is critical for normal hearing. ACh may play a role in neuroregulation of the SMA but several key issues including its membrane action mechanisms remain poorly understood. Besides its well-known endothelium-dependent hyperpolarizing action, ACh can induce a depolarization in vascular cells. Using intracellular and whole-cell recording techniques on cells in guinea pig in vitro SMA, we studied the ionic mechanism underlying the ACh-depolarization and found that: (1) ACh induced a DAMP-sensitive depolarization when intermediate conductance KCa channels were blocked by charybdotoxin or nitrendipine. The ACh-depolarization was associated with a decrease in input resistance (R(input)) in high membrane potential (V(m)) ( approximately -40 mV) cells but with no change or an increase in R input in low Vm ( approximately -75 mV) cells. ACh-depolarization was attenuated by background membrane depolarization from approximately -70 mV in the majority of cells; (2) ACh-induced inward current in smooth muscle cells embedded in a SMA segment often showed a U-shaped I/V curve, the reversal potential of its two arms being near EK and 0 mV, respectively; (3) ACh-depolarization was reduced by low Na+, zero K+ or 20mM K+ bath solutions; (4) ACh-depolarization was inhibited by La3+ in all cells tested, by 4-AP and flufenamic acid in low Vm cells, but was not sensitive to Cd2+, Ni2+, nifedipine, niflumic acid, DIDS, IAA94, linopirdine or amiloride. We conclude that ACh-induced vascular depolarization was generated mainly by activation of a TRP-like non-selective cation channel and by inactivation of an inward rectifier K+ channel.

Molecular mechanisms of early brain injury after subarachnoid hemorrhage

OBJECTIVES

Increasing body of experimental and clinical data indicates that early brain injury after initial bleeding largely contributes to unfavorable outcome after subarachnoid hemorrhage (SAH). This review presents molecular mechanisms underlying brain injury at its early stages after SAH.

METHODS

PubMed was searched using term 'subarachnoid hemorrhage' and key words referring to molecular and cellular pathomechanisms of SAH-induced early brain injury.

RESULTS

The authors reviewed intracranial phenomena and molecular agents that contribute to the early development of pathological sequelae of SAH in cerebral and vascular tissues, including cerebral ischemia and its interactions with injurious blood components, blood-brain barrier disruption, brain edema and apoptosis.

DISCUSSION

It is believed that detailed knowledge of molecular signaling pathways after SAH will serve to improve therapeutic interventions. The most promising approach is the protection of neurovascular unit including anti-apoptosis therapy.

Role of Cl- in cerebral vascular tone and expression of Na+-K+-2Cl- co-transporter after neonatal hypoxia-ischemia

Chloride (Cl-) efflux induces depolarization and contraction of vascular smooth muscle cells. In the basilar arteries from the New Zealand white rabbits, the role of Cl- flux in serotonin-induced contraction was demonstrated by (i) inhibition of Na+-K+-2Cl- co-transporter (NKCC1) to decreased Cl- influx with bumetanide; (ii) a disabled Cl-/HCO3- exchanger with bicarbonate free HEPES solution; (iii) blockade of Cl- channels using 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) and indanyloxyacetic acid 94, R-(+)-methylindazone (R-(+)-IAA-94); and (iv) substitution of extracellular Cl- with methanesulfonate acid (113 mmol/L; Cl-, 10 mmol/L). In addition, the expression of NKCC1 in brain tissues after neonatal hypoxia-ischemia was examined at mRNA and protein levels using RT-PCR and Western blotting techniques. NKCC1 mRNA and protein expressions were increased at 24 and 48 h and returned to normal levels at 72 h after hypoxia insult when compared with the control littermates. In conclusion, Cl- efflux regulates cerebral circulation and the up-regulation of NKCC1 after neonatal hypoxia-ischemia may contribute to brain injury.

Molecular profile of vascular ion channels after experimental subarachnoid hemorrhage

Cerebral vasospasm is a transient, delayed constriction of cerebral arteries that occurs after subarachnoid hemorrhage (SAH). Smooth muscle cells show impaired relaxation after SAH, which may be caused by a defect in the ionic mechanisms regulating smooth muscle membrane potential and Ca(2+) permeability. We tested this hypothesis by examining changes in expression of mRNA and protein for ion channels in the basilar arteries of dogs after SAH using quantitative real-time polymerase chain reaction (PCR) and western blotting. SAH was associated with a significant reduction in basilar artery diameter to 41 +/- 8% of pre-SAH diameter (P < 0.001) after 7 days. There was significant downregulation of the voltage-gated K(+) channel K(v) 2.2 (65% reduction in mRNA, P < 0.001; 49% reduction in protein, P < 0.05) and the beta1 subunit of the large-conductance, Ca(2+) - activated K(+) (BK) channel (53% reduction in mRNA, P < 0.02). There was no change in BK beta1 subunit protein. Changes in mRNA levels of K(v) 2.2 and the BK-beta1 subunit correlated with the degree of vasospasm (r(2) = 0.490 and 0.529 respectively, P < 0.05). The inwardly rectifying K(+) (K(ir)) channel K(ir) 2.1 was upregulated (234% increase in mRNA, P < 0.001; 350% increase in protein, P < 0.001). There was no significant change in mRNA expression of L- type Ca(2+) channels and the BK-alpha subunit. These data suggest that K(+) channel dysfunction may contribute to the pathogenesis of cerebral vasospasm.

Nitric oxide induces hyperpolarization by opening ATP-sensitive K(+) channels in guinea pig spiral modiolar artery

Nitric oxide (NO) hyperpolarizes vascular smooth muscle cells and dilates blood vessels of various beds, but little is known on cochlear vasculatures. Using in vitro preparations of the spiral modiolar artery (SMA), intracellular electrical recording and labeling techniques, we found that the NO donor DPTA-NONOate (10 microM) caused a hyperpolarization of approximately 9 mV in all the cells that had a low resting potential (RP) level near -40 mV. The hyperpolarization amplitude was concentration-dependent, with a 50% effect concentration (EC(50)) of 1 microM. The responses occur in both smooth muscle and endothelial cells, neither of which was blocked by 18beta-glycyrrhetinic acid. The induced hyperpolarization was completely blocked by glipizide, but not by charybdotoxin, apamin, barium, 4-aminopyridine or tetraethylammonium. The hyperpolarizing responses were imitated by pinacidil (EC(50)=30 microM). The pinacidil-induced response was also blocked by glipizide but not by the other K(+) channel blockers mentioned above. Both DPTA-NONOate and pinacidil had little membrane potential effect on cells that had a high RP level near -75 mV. However, when the high RP cells were depolarized to a level beyond -45 mV by barium, both DPTA-NONOate and pinacidil hyperpolarized these cells not differently from those that initially had a low RP. It is concluded that NO hyperpolarizes the SMA primarily by activating K(ATP) channels in both muscle and endothelial cells.

Prevention of cerebral vasospasm by a capsaicin derivative, glyceryl nonivamide, in an experimental model of subarachnoid hemorrhage

BACKGROUND

Cerebral vasospasm after aneurysmal subarachnoid hemorrhage (SAH) remains a major complication in patients suffering from SAH. In our previous study, we reported that stimulating vascular K(+) channel activity prevented the development of cerebral vasospasm. Recent evidence indicates that glyceryl nonivamide (GLNVA), a capsaicin derivative, has a vasorelaxant effect on the aortic vascular smooth muscle due to the release of coronary calcitonin gene-related peptide, which in turn stimulates K(+) channel opening. The purpose of the present study was to examine the preventive effects of GLNVA on vasospasm.

METHODS

New Zealand white rabbits were subjected to experimental SAH by injecting autologous blood into the cisterna magna. GLNVA or vehicle was injected intrathecally immediately after the induction of SAH. All animals were killed by perfusion-fixation at 48 hours after SAH. The basilar arteries were removed and sectioned, and their cross-sectional areas were measured.

RESULTS

The average cross-sectional areas of basilar arteries were reduced by 69% and 71% in the SAH only and SAH plus vehicle groups, respectively, when compared with the healthy controls. After treatment with 0.35, 1.75, and 3.5 mg/kg GLNVA in rabbits subjected to SAH the average cross-sectional area was decreased by 46%, 12% and 2%, respectively, when compared with the healthy controls. The protective effect of GLNVA achieved statistical significance at all dosages. Morphologically, corrugation of the internal elastic lamina of vessels was often observed in the vehicle-treated group, but was not prominent in the GLNVA-treated groups or healthy controls.

CONCLUSION

The findings showed that GLNVA dose-dependently attenuated cerebral vasospasm after SAH in the rabbit. These results suggest that intrathecal administration of GLNVA could be an effective strategy for preventing cerebral vasospasm after SAH.

The presence of an extractable substance in the CSF of humans with cerebral vasospasm after subarachnoid haemorrhage that correlates with phosphatase inhibition

The cellular events leading to cerebral vasospasm after subarachnoid haemorrhage are poorly understood, although an increase in smooth muscle myosin light chain phosphorylation has been observed. This study set out to determine if phosphatase inhibition may be involved in the pathological maintenance of tension observed during vasospasm. We found that 1 nM okadaic acid, a type 2A protein phosphatase inhibitor, elicited an increase in rate of O(2) consumption in the porcine carotid artery similar to that by cerebrospinal fluid (CSF) from vasospastic patients (CSF(V), n=5) (control 0.23+/-0.03, CSF(V) 0.84+/-0.16 and okadaic acid 0.85+/-0.02 micromol min(-1) g dwt(-1)). It was also observed that phosphatase inhibition with 1 nM okadaic acid significantly slowed relaxation after a stretch in a similar fashion to CSF(V) haemorrhage. CSF from vasospastic subarachnoid haemorrhage patients, but not from those without vasospasm, contains an extractable substance which modulates myosin light chain phosphorylation in vitro. A phosphatase preparation obtained from the porcine carotid artery dephosphorylated 63+/-2% of the phosphorylated (MLC(20)) substrate in vitro, and non-vasospastic CSF treated enzyme dephosphorylated 60+/-2.6%. Okadaic acid inhibited phosphatase dephosphorylated only 7.5+/-1% of the substrate where CSF(V) treated enzyme dephosphorylated 22+/-2.8% of the substrate. We conclude that inhibition of smooth muscle phosphatase may be involved in the mechanisms associated with cerebral vasospasm after subarachnoid haemorrhage.

Levcromakalim decreases cytoplasmic Ca2+ and vascular tone in basilar artery of SAH model dogs

We investigated the effects of levcromakalim, a K+ channel opener, on [Ca2+]i and the contractile force of basilar arteries obtained from normal dogs and subarachnoid hemorrhage (SAH) dogs. The responsiveness to serotonin was increased more in the SAH group than in the control group. Levcromakalim decreased the resting [Ca2+]i and force more profoundly than did a Ca2+ channel blocker, nicardipine, and these effects were more prominent in the SAH group than in the control group. Levcromakalim diminished the increases in [Ca2+]i and contractile force induced by serotonin more profoundly than nicardipine did, and these effects were equal in both groups. The effects of levcromakalim were not inhibited by iberiotoxin but were antagonized completely by glibenclamide. These results suggest that levcromakalim-induced opening of adenosine triphosphate (ATP)-sensitive K+ (K(ATP)) channels reduces [Ca2+]i more effectively than does nicardipine and that levcromakalim exerts the vasodilator effects under the condition in which large conductance Ca2+-activated K+ (BK) channels are blocked with iberiotoxin. Consequently, K+ channel openers like levcromakalim may be useful drug candidates to treat delayed cerebral vasospasm after SAH.

Levcromakalim decreases vascular tone, cytoplasmic Ca2+ and Ca2+ sensitivity in canine basilar artery

The involvement of large conductance Ca(2+)-activated K+ channels (BK) and ATP-sensitive K+ (KATP) channels in the regulation of canine basilar arterial tone was estimated in the presence of the agonist and blockers of these channels, by simultaneously measuring the changes in intracellular Ca2+ concentration ([Ca2+]i) with the fura-2 microfluorimetric method. In the resting condition, levcromakalim reduced [Ca2+]i and vascular tone. Levcromakalim suppressed the serotonin-induced increases in [Ca2+]i and force of contraction, the maximum effects of which were much greater than those of nicardipine. The inhibitory effects of levcromakalim were blocked by glibenclamide but not by tetraethylammonium (TEA) or iberiotoxin (IbTX). In the presence of levcromakalim, the curve relating [Ca2+]i with force in the presence of serotonin at different extracellular Ca2+ concentration ([Ca2+]o) was shifted down- and right-ward compared with that in the absence of levcromakalim, suggesting that levcromakalim may reduce the Ca(2+)-sensitivity of the contractile proteins. Thus, levcromakalim may be a good candidate to suppress delayed cerebral vasospasm after subarachnoid hemorrhage.

Systemic administration of the potassium channel activator cromakalim attenuates cerebral vasospasm after experimental subarachnoid hemorrhage

OBJECTIVE

Cerebral vasospasm is a primary complication after aneurysmal subarachnoid hemorrhage (SAH). Recent evidence indicates that the activation of potassium (K+) channels may be of benefit in relieving spastic constriction. The present study examined the effects of systemic administration of a K+ channel activator, cromakalim, on cerebral vasospasm after experimental SAH.

METHODS

Experimental SAH was performed in rabbits by injecting autologous blood into the cisterna magna. Intravenous injections of cromakalim or vehicle were administered twice daily with the first injection administered 1 hour after induction of SAH. Animals were killed by perfusion-fixation 48 hours after SAH. Basilar arteries were removed and sectioned, and the luminal cross-sectional areas were measured.

RESULTS

Experimental SAH induced cerebral vasospasm in untreated and vehicle-treated animals. Cromakalim attenuated cerebral vasospasm in a dose-dependent manner. This effect achieved statistical significance at doses of 0.1 and 0.3 mg/kg.

CONCLUSION

These results support the concept that targeting vascular K+ channels can be of benefit in preventing the development of cerebral vasospasm. The findings also indicate that cromakalim represents a potential therapeutic agent for the treatment of cerebrovascular pathophysiology after SAH.

Characterization of the potassium channels involved in EDHF-mediated relaxation in cerebral arteries

1. In the presence of NG-nitro-L-arginine (L-NOARG, 0.3 mM) and indomethacin (10 microM), the relaxations induced by acetylcholine and the calcium (Ca) ionophore A23187 are considered to be mediated by endothelium-derived hyperpolarizing factor (EDHF) in the guinea-pig basilar artery. 2. Inhibitors of adenosine 5'-triphosphate (ATP)-sensitive potassium (K)-channels (KATP; glibenclamide, 10 microM), voltage-sensitive K-channels (Kv; dendrotoxin-1, 0.1 microM or 4-aminopyridine, 1 mM), small (SKCa; apamin, 0.1 microM) and large (BKCa; iberiotoxin, 0.1 microM) conductance Ca-sensitive K-channels did not affect the L-NOARG/indomethacin-resistant relaxation induced by acetylcholine. 3. Synthetic charybdotoxin (0.1 microM), an inhibitor of BKCa and Kv, caused a rightward shift of the concentration-response curve for acetylcholine and reduced the maximal relaxation in the presence of L-NOARG and indomethacin, whereas the relaxation induced by A23187 was not significantly inhibited. 4. A combination of charybdotoxin (0.1 microM) and apamin (0.1 microM) abolished the L-NOARG/ indomethacin-resistant relaxations induced by acetylcholine and A23187. However, the acetylcholine-induced relaxation was not affected by a combination of iberiotoxin (0.1 microM) and apamin (0.1 microM). 5. Ciclazindol (10 microM), an inhibitor of Kv in rat portal vein smooth muscle, inhibited the L-NOARG/ indomethacin-resistant relaxations induced by acetylcholine and A23187, and the relaxations were abolished when ciclazindol (10 microM) was combined with apamin (0.1 microM). 6. Human pial arteries from two out of four patients displayed an L-NOARG/indomethacin-resistant relaxation in response to substance P. This relaxation was abolished in both cases by pretreatment with the combination of charybdotoxin (0.1 microM) and apamin (0.1 microM), whereas each toxin had little effect alone. 7. The results suggest that Kv, but not KATP and BKCa, is involved in the EDHF-mediated relaxation in the guinea-pig basilar artery. The synergistic action of apamin and charybdotoxin (or ciclazindol) could indicate that both Kv and SKCa are activated by EDHF. However, a single type of K-channel, which may be structurally related to Kv and allosterically regulated by apamin, could also be the target for EDHF.

Modulation by the endothelium of the inhibitory effects of pinacidil and nimodipine on endothelin-induced contraction in cerebral arteries

The effects of pinacidil and nimodipine on endothelin-1-induced contractions in isolated cerebral arteries with and without endothelium were compared. The sensitivity to endothelin-1 was increased (0.5 log units) in the rabbit basilar artery after removal of the endothelium. The nitric oxide synthase inhibitor N omega-nitro-L-arginine (0.1 mM) also increased the sensitivity to endothelin-1 (0.6 log units) in basilar arteries with endothelium, whereas N omega-nitro-D-arginine (0.1 mM) and indomethacin (3 microM) had no effect, indicating that withdrawal of endothelium-derived nitric oxide may account for the enhancement of the endothelin-1-induced contraction after endothelial denudation. Pinacidil (1 microM) shifted the concentration-response curve for endothelin-1 to the right without affecting the maximal response in arteries without endothelium, but had no effect on the endothelin-1-induced contraction in vessels with endothelium. Nimodipine (1 microM) reduced the maximal endothelin-1-induced contraction by approximately 50% in both the presence and absence of endothelium, whereas the sensitivity to endothelin-1 was reduced only in vessels without endothelium. Incubation in "calcium-free" medium reduced the maximal endothelin-1-induced contraction by 69% and 80% in vessels with and without endothelium, respectively. In human pial arteries with endothelium, pinacidil did not affect the endothelin-1-induced contraction, whereas nimodipine and exposure to "calcium-free" solution reduced the maximal response by 31% and 74% respectively. The results show that, in the rabbit, pinacidil and to a lesser extent nimodipine preferentially act on cerebral arteries with disrupted endothelium, indicating that vasoactive factors liberated from the endothelium may modify the effect of a vasodilator.

The pharmacological properties of K+ currents from rabbit isolated aortic smooth muscle cells

1. Using the whole-cell patch-clamp technique, the effects of several K+ channel blocking drugs on K+ current recorded from rabbit isolated aortic smooth muscle cells were investigated. 2. Upon depolarization from -80 mV, outward K+ current composed of several distinct components were observed: a transient, 4-aminopyridine (4-AP)-sensitive component (I1) and a sustained component (Isus), comprising a 4-AP-sensitive delayed rectifier current (IK(V)), and a noisy current which was sensitive to tetraethylammonium (TEA), and probably due to Ca(2+)-activated K+ current (IK(Ca)). 3. Several drugs in clinical or experimental use have as part of their action an inhibitory effect on specific K+ channels. Because of their differential K+ channel blocking effects, these drugs were used in an attempt to characterize further the K+ channels in rabbit aortic smooth muscle cells. Imipramine, phencyclidine, sotalol and amitriptyline failed to block selectively any of the components of K+ current, and were thus of little value in isolating individual channel contributions. Clofilium showed selective block of IK(V) in the presence of TEA, but only at low stimulation frequencies (0.07 Hz). At higher frequencies (1 Hz) of depolarization, both I1 and IK(V) were suppressed to a similar extent. Thus, the blocking action of clofilium was use-dependent. 4. The voltage-dependent inactivation of I1 and the delayed rectifier were very similar although a brief (100 ms) pre-pulse to -30 mV could preferentially inactivate I1. Together with the non-selective blocking effects of the K+ channel blockers, similarities in the activation and inactivation of these two components suggest that they may not exist as separate ionic channels, but as distinct kinetic states within the same K+ channel population. 5. The effects of all of these drugs on tension were examined in strips of rabbit aorta. The non-specific K+ channel blockers caused only minor increases in basal tension. TEA and 4-AP by themselves caused significant increases in tension and were even more effective when applied together. There appeared to be no correlation between the effects of the drugs tested on tension and their actions on currents recorded from isolated myocytes. Thus tension studies are an inappropriate means of investigating the mechanism of action of these drugs, and studies on ionic currents in isolated myocytes cannot easily predict drug actions on intact tissues.