The control of carrier frequency in cricket calls: a refutation of the subalar-tegminal resonance/auditory feedback model.

The subalar-tegminal resonance/auditory feedback hypothesis attempts to explain how crickets control the carrier frequency (f(C)), the loudness and the spectral purity of their calls. This model contrasts with the 'clockwork cricket' or escapement model by proposing that f(C) is not controlled by the resonance of the cricket's radiators (the harps) but is instead controlled neurally. It suggests that crickets are capable of driving their harps to vibrate at any frequency and that they use a tunable Helmholtz-like resonator consisting of the tegmina and the air within the subalar space to amplify and filter the f(C). This model predicts that f(C) is variable, that call loudness is related to tegminal position (and subalar volume) and that low-density gases should cause f(C) to increase. In Anurogryllus arboreus, f(C) is not constant and varied by as much as 0.8 % between pulses. Within each sound pulse, the average f(C) typically decreased from the first to the last third of a sound pulse by 9 %. When crickets called in a mixture of heliox and air, f(C) increased 1.07- to 1.14-fold above the value in air. However, if the subalar space were part of a Helmholtz-like resonator, then its resonant frequency should have increased by 40-50 %. Moreover, similar increases occurred in species that lack a subalar space (oecanthines). Experimental reduction of the subalar volume of singing crickets resulted neither in a change in f(C) nor in a change in loudness. Nor did crickets attempt to restore the subalar volume to its original value. These results disprove the presence of a subalar-tegminal resonator. The free resonance of freshly excised Gryllus rubens tegmina shifted by 1.09-fold when moved between air and a mixture of helium and air. Auditory feedback cannot be the cause of this shift, which is similar to the f(C) shifts in intact individuals of other species. Calculations show that the harp is 3.9-1.8 times more massive than the air that moves en masse with the vibrating harps. Replacing air with heliox-air lowers the mass of the vibrating system sufficiently to account for the f(C) shifts. These results re-affirm the 'clockwork cricket' (escapement) hypothesis. However, as realized by others, the harps should be viewed as narrow-band variable-frequency oscillators whose tuning may be controlled by factors that vary the effective mass.

Cardiac maturation in an hypoxic milieu: implications for arrhythmias in hypoxemic defects.

Newborn rabbits were raised in either hypoxic or normoxic environments for 3 months. Data from electrophysiological studies carried out on isolated sinoatrial and ventricular preparations from the two groups show that the hypoxic group had changes in cellular transmembrane electrical activity that could be the basis for disorders of rate and conduction in vivo. This model will be useful to study the etiology and therapy of some arrhythmias seen in cyanotic congenital hypoxemic heart disease.

A new method for raising neonatal rabbits in a hypoxic environment.

An apparatus and method for rearing neonatal rabbits in hypoxia is described. This technique relies on the use of hypoxia chambers that need to be serviced once a day for approximately 1 h. By use of the apparatus and procedures outlined, rabbits that exhibit standard clinical signs of hypoxemia (cyanosis and elevated hematocrit) can be reliably reared and maintained for long periods of time.

Electrophysiologic effect of indomethacin on the rabbit sinus node.

The anti-inflammatory agent indomethacin has been shown to affect calcium in smooth muscle, cardiac muscle, and cardiac Purkinje cells. This study looked at the action of indomethacin on rabbit sinus node. High doses of indomethacin reduce the slope of diastolic depolarization, spontaneous rate, action potential amplitude and time of repolarization. These results are consistent with a reduction of inward transmembrane movement of calcium.

The resistance of the cardiac muscarinic receptor to chronic ethanol ingestion in the rat.

The chronic consumption of alcohol has been correlated with the development of cardiomyopathy and dysrhythmias. These disorders may be secondary to changes in the outflow of the autonomic nervous system. This study looked at the changes in cardiac cell responsiveness to carbamylcholine (CBL) and the concentration of muscarinic receptors. The studies were carried out on hearts excised from male Long-Evans hooded rats, pair-fed on ethanol (E) or control (C) liquid diet. The ethanol diet supplied 35-39% of calories as ethanol. The studies were carried out after 8-10 wks or 18-20 wks on the diet. Ventricular muscarinic receptors were measured using (-)-[3H]quinuclidinyl benzilate (QNB). These studies showed no significant difference in muscarinic receptor concentration or the dissociation constant for [3H]QNB binding after 8-10 wks or 18-20 wks of E ingestion. Electrophysiologic studies of chronotropic responsiveness to CBL showed no significant difference between the E and C groups after 8-10 wks or 18-20 wks. The responsiveness of subsidiary pacemaker cells to CBL was not different between E and C groups at 18-20 wks. These data indicate that chronic E ingestion for up to 20 wks did not effect the cardiac muscarinic receptor concentration or cholinergic response.

The activities of enzymes associated with anaerobic pathways, glycolysis and the Krebs cycle in spiders.

1. The activities of eight enzymes that participate in or are closely linked to anaerobic glycolysis or the krebs cycle were measured in 11 species of spiders representing diverse taxonomic groups (Table 1). 2. The results indicate three anaerobic pathways may operate in spiders. The major pathway leads to the accumulation of D-lactate and one of the minor pathways to alanine and malate as postulated by Linzen & Gallowitz (1975). Another minor pathway ends with the accumulation of glycerol-3-phosphate and pyruvate (Fig 1). 3. There were no consistent correlations between the degree of development of certain pathways or enzyme activities and a spider's habits and respiratory exchange system. 4. Lactate pathways are much more highly developed in the legs and cephalothorax than in the abdomen.