A novel effect of cobalt treatment on calcium-dependent responses of the cockroach salivary gland

The effects of manganese, cobalt and calcium on amylase secretion and calcium homeostasis in rat pancreas

1. Mn(2+) evoked an atropine-resistant secretion of amylase from the isolated pancreas of the young rat. The lowest effective concentration of Mn(2+) was 10(-3)m. The response to 10(-2)m-Mn(2+) was biphasic, an initial peak being followed by a slow sustained rise in amylase output. The maximal effect of 10(-2)m-Mn(2+) was to double the basal rate of amylase secretion after 70 min incubation.2. Co(2+) (10(-2)m) also stimulated amylase secretion. The maximal rate, about three times the basal value, was attained after 20 min incubation. Atropine partially inhibited this effect.3. Ca(2+) (10(-2)m) evoked an atropine-resistant amylase secretion similar in both magnitude and time course to the sustained phase observed with 10(-2)m-Mn(2+).4. Mn(2+) (10(-4)-10(-2)m) also increased the rate of (45)Ca efflux from the gland. Maximal efflux rates were attained after 30 min incubation and thereafter declined to basal values. A small increase was also observed with 10(-2)m-Co(2+), but not with 10(-2)m-Ca(2+). The effect of Co(2+) was almost completely abolished by atropine.5. Reducing the extracellular Ca(2+) concentration from 2.5 x 10(-3) to 10(-5)m did not reduce amylase secretion in response to 10(-2)m-Mn(2+), but secretion was abolished in a Ca(2+)-free medium containing EGTA. The increase in (45)Ca efflux rate evoked by Mn(2+) was inversely related to the extracellular Ca(2+) concentration.6. Mn(2+) (10(-2)m) increased the concentration of cyclic 3',5'-guanosine monophosphate (cyclic GMP) within the pancreas. Also, Mn(2+) accumulated within the cellular pool of the gland. The time course of both these effects was similar to the time course of (45)Ca efflux.7. Mn(2+) displaced Ca(2+) bound to isolated pancreatic microsomal membranes. The cation-binding sites on these membranes probably have a higher affinity for Mn(2+) than Ca(2+).8. We conclude that Mn(2+) stimulates enzyme secretion by displacing membrane-bound Ca(2+), the resulting increase in cytosolic Ca(2+) concentration activating the secretory mechanism.9. Mn(2+) partially inhibited amylase secretion stimulated by optimal doses of either acetylcholine (ACh) or caerulein. Maximal inhibition (about 60%) occurred with 10(-3)m-Mn(2+) (i.e. the lowest concentration required to stimulate secretion in the absence of secretagogues). Decreasing the extracellular Ca(2+) concentration reduced the inhibitory effect of Mn(2+).10. When glands were exposed to ACh and Mn(2+) simultaneously, the time required for inhibitory effects to develop was inversely related to the dose of ACh and the concentration of Mn(2+).11. Mn(2+) did not alter the acceleration of (45)Ca efflux evoked by ACh or by caerulein in a medium containing 2.5 x 10(-3)m-Ca(2+). However, under conditions of Ca(2+) deprivation ACh-stimulated (45)Ca efflux was greatly enhanced.12. Mn(2+) reduced the total amount of Ca(2+) accumulated into the cellular pool of the pancreas after 60 min incubation, but had no effect on the initial, rapid phase of Ca(2+) uptake.13. The effects of Mn(2+) on the relationship between ACh dose, amylase release and the extracellular Ca(2+) concentration suggest that the inhibitory actions of Mn(2+) cannot be explained by a simple, competitive interaction with the stimulant or with extracellular Ca(2+). However, the time course of inhibition is consistent with a requirement for Mn(2+) to accumulate within the acinar cells.14. Mn(2+) partially inhibited amylase secretion stimulated by hyperosmolarity and also increased the (45)Ca efflux rate under these conditions.15. Our results are not consistent with Mn(2+) exerting its inhibitory effect on secretagogue-stimulated enzyme secretion solely by blocking Ca(2+) influx from the extracellular space. We conclude that inhibition probably depends on the ability of Mn(2+) to displace Ca(2+) from binding sites involved in secretion, presumably coupled with a reduced ability of Mn(2+) to replace Ca(2+) in the secretory process.

A calcium-readmission response recorded from Nauphoeta salivary gland acinar cells

1. When cockroach salivary glands are exposed to bathing solutions without added calcium, reintroduction of calcium causes the acinar cells to hyperpolarize. The effect ('readmission response') may be very prolonged if the conditioning solution contains 5 mM-cobalt. 2. Evidence is presented against the possibility that the readmission response is mediated by transmitter released from the salivary nerves. 3. The readmission response is shown to reflect an increase in potassium conductance.

On the role of calcium in the electrical responses of cockroach salivary gland cells to dopamine

1. The calcium dependence of the intracellularly recorded hyperpolarizing responses of salivary gland acinar cells of Nauphoeta cinerea (Olivier) to ionophoretically applied dopamine has been examined. The results of withdrawing calcium from the bathing solution were essentially the same whether or not other divalent cations were present. The effects of calcium withdrawal were rapidly reversed on replacement of calcium. 2. Small responses to dopamine were reduced or abolished by calcium withdrawal but could be restored by an increase in the amount of ejected dopamine. Calcium withdrawal did not have any consistent effect on the input resistance: a reduction in input resistance is therefore not the main cause of the reduction in the amplitude of the responses to dopamine. 3. In very low calcium solutions responses elicited by repeated prolonged dopamine applications progressively declined. 4. It is suggested that the hyperpolarizing responses to dopamine depend on an influx of calcium into the cytosol from a store which can be replenished only from the exterior.