Male weanling swine were injected daily for up to 14 days with the organophosphate cholinesterase inhibitors, diisopropylfluorophosphate (DFP) or sarin. The clinical signs of poisoning disappeared or were attenuated by 7 days after starting the DFP treatment, indicating the development of tolerance to DFP toxicity. 2. A significant decrease in acetylcholinesterase activity (85-98%) occurred over the course of this treatment followed by a decrease in the maximal density (Bmax) of [3H] quinuclidinyl benzilate ([3H]QNB) and [3H] N-methylscopolamine ([3H]-NMS) binding sites in isolated cells. The affinity of the muscarinic receptors (KD) for [3H]QNB and [3H]NMS binding, however, remained unaffected. 3. Dose-response curves for ACh-induced increase in isometric tension of tracheal smooth muscle (TSM) showed a leftward shift from control, 2 h after DFP injection. Twenty-four hours after the last DFP treatment, for animals receiving 1 or up to 14 daily injections of DFP, all the dose-response curves were shifted to the left to approximately the same ACh sensitivity when compared with that for control tissue. 4. In vitro treatment of the muscle with 10(-4) M DFP shifted the dose-response curves leftward, in both control and injected animals, and rendered the muscles from control, 1- and 3-day injected animals sensitive to ACh concentrations as low as 10(-10) M. Sensitivity to 10(-10) M ACh was eliminated by carefully cleaning the smooth muscle of adherent connective tissue containing nerves and ganglia and after subacute treatment of swine for 7 days with DFP. DFP-induced spontaneous contractions were also eliminated by careful cleaning. 5. Subacute DFP treatment caused a small leftward shift in the dose-response curve for bethanechol at 2 h and a rightward shift at 1,3 and 7 days, compared to controls. 6. Dose-response curves for K+ were shifted to the right after 1 and 3 days of DFP treatment, but shifted back towards the control after 7 days of treatment. The muscle cells were hyperpolarized by approximately 5 mV after 7 days of DFP or sarin injections. The membrane potential was slightly more sensitive to changes in K+ concentration after 7 days of sarin injection. 7. Subacute treatment of swine with organophosphates modifies the response of neural elements in swine TSM to ACh. Chronic cholinesterase inhibition causes a reduction in the sensitivity of the neural elements to ACh. The decrease in muscarinic receptor density which occurs with chronic cholinesterase inhibition is not sufficient to explain tolerance to organophosphates since TSM maintains an almost normal responsiveness to ACh.
