Pulmonary mechanoreceptors in the dipnoi lungfish Protopterus and Lepidosiren

Pulmonary mechanoreceptors in the dipnoi lungfish Protopterus aethiopicus and Lepidosiren paradoxa were identified and characterized both in vivo and in vitro. Both slowly adapting receptors (SAR) and rapidly adapting receptors (RAR) were found, with the majority of receptors being of the slowly adapting type. The SAR discharged during inflation once their threshold volume was exceeded, and this activity persisted during prolonged lung inflation. For most of the SAR, interaction between the rate of inflation and lung volume was involved in setting the firing rate of the receptor. Increase in intrapulmonary carbon dioxide partial pressure decreased the activity of the SAR during sustained inflation at a constant intrapulmonary pressure. The RAR also discharged during inflation once the threshold volume was exceeded. However, in contrast to the SAR, the RAR became silent within 1.5 - 15 s after the end of inflation. The activity of these receptors during inflation was influenced predominantly by the rate and influenced little by the volume of inflation. According to these observations, the behavior of lungfish pulmonary mechanoreceptors closely resembles that of pulmonary mechanoreceptors in amphibians but differs from that of mechanoreceptors in the swim bladder of teleosts.