The effect of verapamil and EGTA on the rat phrenic nerve-hemidiaphragm preparation

Relatively high concentrations of verapamil or EGTA [ethylene glycol-bis (beta-aminoethyl ether) N, N, N',N'-tetra acetic acid] inhibit contraction (P) of the rat phrenic nerve-hemidiaphragm preparation elicited by direct or indirect stimulation. The inhibitory effect of verapamil is greater (P less than 0.002) with direct (I50 = 26.3 +/- 1.7 microM) than indirect = I50 = 37.6 +/- 1.9 microM) stimulation. For EGTA the reverse is true: I50 is 1320 +/- 80 microM with direct and 1100 +/- 60 microM with indirect stimulation. The greater than 90% verapamil-induced depression of P can only be partially reversed by washout. Increasing the [Ca2+] or the addition of 4-aminopyridine (4AP) has insignificant antagonist effect. Except for the antagonism by 4AP during direct stimulation, the EGTA-induced depression of P is better antagonized by washout, increase of the [Ca2+], or the addition of 4AP than that caused by verapamil. Neostigmine did not antagonize the depression of P caused by either verapamil or EGTA. The findings presented indicate that the primary site of action of verapamil is postjunctional and that of EGTA is prejunctional.