Streptomycin inhibition of myogenic tone, K+-induced force and block of L-type calcium current in rat cerebral arteries

Abstract

1. Streptomycin has been demonstrated to inhibit mechanosensitive conductances in a wide variety of cell types, including muscle. The action of streptomycin on rat cerebral arteries that exhibit pressure-induced myogenic response was investigated. 2. Pressure-induced tone, measured using isobaric myography, in isolated pressurized cerebral arteries was reversibly and concentration-dependently inhibited by streptomycin with an IC50 of 2.6 mM. 3. Isometric K+-induced force, measured using isometric myography, is supported by voltage-gated Ca2+ entry. Streptomycin reversibly and concentration-dependently inhibited isometric force with an IC50 of 1.71 mM. 4. Voltage-gated macroscopic inward Ca2+ channel currents were recorded from freshly isolated rat basilar myocytes. These were reversibly and concentration-dependently inhibited by streptomycin with an IC50 of 1.79 and 0.47 mM when 10 and 1.8 mM CaCl2, respectively, was used as the charge carrier. 5. These data suggest that streptomycin inhibits myogenic tone and K+-induced isometric force largely by blockade of L-type, dihydropyridine-sensitive Ca2+ channels. In conclusion, streptomycin is not useful in the investigation of stretch-activated channels which may underlie the myogenic response of rat small cerebral arteries.

Figure 1. Effect of streptomycin on pressure-induced tone

A, diameter changes in a section of artery held at a constant 80 mmHg during an increase in the superfusion temperature from 22 to 37 °C. B, diameter changes in response to increasing concentrations of streptomycin (indicated by horizontal bars) surrounding a pressurized (80 mmHg) vessel. The DHP antagonist (-)202–791 (2 μM) was added to establish the passive diameter and to confirm the dependence of tone on DHP-sensitive Ca²⁺ channels. C, mean concentration-effect data of streptomycin inhibition of blood vessel diameter from 5 vessels fitted with a logistic curve giving an IC₅₀ of 2.6 ± 0.33 mM and a slope of 1.88 ± 0.46.

Figure 2. Effect of streptomycin on K⁺-induced isometric force

A shows a typical recording of isometric force in response to K⁺-induced depolarization and subsequent relaxation to cumulative addition of streptomycin (arrows) followed by recovery of force on washout. B, normalized mean concentration-effect data for streptomycin inhibition of K⁺-induced force from 12 vessels, fitted with a logistic curve giving an IC₅₀ value of 1.71 ± 0.26 mM and a slope of 1.69 ± 0.19.

Figure 3. Effect of streptomycin on voltage-gated Ca²⁺ current

A, representative inward currents recorded from a single isolated basilar smooth muscle cell (10 mM Ca²⁺ bathing solution) in response to a 200 ms step depolarization applied at 15 s intervals from -80 to +10 mV before (Control) and after the addition of different concentrations of streptomycin (arrows). B, mean normalized concentration-effect data for streptomycin inhibition of inward current using 10 mM (•) and 1.8 mM (□) Ca²⁺ as the charge carrier fitted with a logistic curve giving mean IC₅₀ values (±s.e.m.) of 1.79 ± 0.4 and 0.47 ± 0.13 mM with slope factors (±s.e.m.) of 0.93 ± 0.1 and 0.85 ± 0.1 from 5 and 4 cells, respectively. C, mean I-V relations for Ca²⁺ current recorded in 10 mM CaCl₂ from 3 cells in the absence (♦) and presence (○) (±s.e.m.) of 3 mM streptomycin. Inward current was elicited from a holding potential of -80 mV at 6 s intervals and measured as the difference in current in the presence and absence of 2 mM Co²⁺.