ISSN:
1432-1912
Keywords:
Nifedipine
;
Mammalian cardiac muscle
;
Amplitude-frequency relationship
;
Staircase phenomena
;
Transmembrane action potential
;
SA-nodal rate
Source:
Springer Online Journal Archives 1860-2000
Topics:
Medicine
Notes:
Summary 1. The effects of nifedipine (3×10−11–3×10−6 M) on contractile activity and transmembrane action potential (AP) were studied in isotonically contracting cat papillary muscles and in isolated SA-nodes. 2. Nifedipine (3×10−8–3×10−6 M) reduces the amplitudes of contraction. The effect is nearly independent of stimulation rate such that at 6/min the ED50 amounts to 6.1×10−7 M as compared to 5.1×10−7 M at 60/min. This negative inotropic action can be nearly completely antagonized by raising [Ca2+] o . 3. Nifedipine induces short-lasting negative stair-cases developing upon step increases of stimulation rate. Raising [Ca2+] o does not counteract this effect. 4. Nifedipine exerts a dual action on the amplitudeinterval relationship for simple test contractions elicited after a variable period of rest and preceding conditioning stimulation: (i) a flattening of the initial part of the “restitution” curve and (ii) a depression of the steady-state level reached after 2–10 s of rest. Raising [Ca2+] o elevates the steady state to control level but does not reestablish the initial slope of the curve. 5. Nifedipine, while leaving resting potential, maximum rate of depolarization and overshoot unaffected, tends to abbreviate the AP duration. Since this effect is more pronounced at lower frequencies, the usual prolongation of the AP occurring at low stimulation rates is inhibited. The AP plateau is depressed, but this effect seems to be stronger at higher stimulation rates. 6. In isolated SA nodes nifedipine (3×10−7–1×10−6 M) decreases discharge rate by reducing both the rate of the slow diastolic depolarization and the maximal diastolic potential until intranodal conduction blocks occur. 7. It is concluded that the negative inotropic action of nifedipine results mainly from diminished transmembrane Ca supply, with a slight action on internal sites of cardiac e.-c. coupling.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00501261
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