Synthesis and secretion of insulin in dynamic perfusion systems

Plasma native and peptidase-derivable Met-enkephalin responses to restraint stress in rats. Adaptation to repeated restraint

Met-enkephalin and related proenkephalin A-derived peptides circulate in plasma at picomolar concentration as free, native pentapeptide and at nanomolar concentration in cryptic forms. We have optimized conditions for measurement of immunoreactive Met-enkephalin in plasma and for generation by trypsin and carboxypeptidase B of much greater amounts of total peptidase-derivable Met-enkephalin in plasma of rats, dogs, and humans. Free Met-enkephalin (11 pM) is constituted by native pentapeptide and its sulfoxide. Characterization of plasma total Met-enkephalin derived by peptidic hydrolysis revealed a small amount (38 pM) of Met-enkephalin associated with peptides of molecular mass less than 30,000 D, and probably derived from proenkephalin A, but much larger amounts of Met-enkephalin associated with albumin (1.2 nM) and with a globulin-sized protein (2.8 nM). Thus, plasma protein precursors for peptidase-derivable Met-enkephalin differ structurally and chemically from proenkephalin A. Met-enkephalin generated from plasma by peptidic hydrolysis showed naloxone-reversible bioactivity comparable to synthetic Met-enkephalin. Prolonged exposure of adult, male rats to restraint stress produced biphasic plasma responses, with peaks occurring at 30 s and 30 min in both free native and total peptidase-derivable Met-enkephalin. Repeated daily exposure to this 30-min stress resulted in adaptive loss of responses of both forms to acute restraint. Initial plasma responses of Met-enkephalin paralleled those of epinephrine and norepinephrine, but subsequently showed divergence of response. In conclusion, Met-enkephalin circulates in several forms, some of which may be derived from proteins other than proenkephalin A, and plasma levels of both free native, and peptidase-derivable Met-enkephalin are modulated physiologically.

Insulin secretory dynamics after two consecutive intravenous stimulations with glucose and/or tolbutamide

Intravenous glucose and/or tolbutamide administered in two consecutive pulses 30 and 60 min apart to the same subjects using identical doses showed that insulin secretory responses was altered during a subsequent stimulation and that this was modulated by the time factor. Insulin response was more sustained after the second glucose pulses and the insulin peak response was delayed and diminished if the second glucose dose was given 30 min after the first, but not if given 60 min later. It is suggested that the beta-cell membrane might remain partially depolarized above a certain glucose level or that a postulated signal relay mechanism might become saturated. Responses to two tolbutamide pulses did not show these characteristics; however, the second insulin response was smaller than the first. When the first pulse was glucose and the second tolbutamide, or vice versa, the second responses were altogether different from those elicited by the double doses of either tolbutamide or glucose. To explain these characteristic patterns of insulin secretory dynamics, the existence of occult glucose receptors on the beta-cell that are opened up by tolbutamide was postulated. These studies do not support the two-pool theory, or at least restrict it to glucose-stimulated insulin response. The positive correlations between the first and the second insulin responses in all tests argue strongly against the existence of an insulin feedback mechanism in man.